diff --git a/src/modules/user/application/dtos/OAuthCallbackDTO.ts b/src/modules/user/application/dtos/OAuthCallbackDTO.ts
new file mode 100644
index 00000000..3b5a298b
--- /dev/null
+++ b/src/modules/user/application/dtos/OAuthCallbackDTO.ts
@@ -0,0 +1,4 @@
+export interface OAuthCallbackDTO {
+  code: string;
+  state: string;
+}
diff --git a/src/modules/user/application/dtos/TokenDTO.ts b/src/modules/user/application/dtos/TokenDTO.ts
new file mode 100644
index 00000000..366769d6
--- /dev/null
+++ b/src/modules/user/application/dtos/TokenDTO.ts
@@ -0,0 +1,11 @@
+export interface TokenPair {
+  accessToken: string;
+  refreshToken: string;
+  expiresIn: number;
+}
+
+export interface TokenPayload {
+  did: string;
+  iat: number;
+  exp: number;
+}
diff --git a/src/modules/user/application/dtos/UserDTO.ts b/src/modules/user/application/dtos/UserDTO.ts
new file mode 100644
index 00000000..e04e0f5e
--- /dev/null
+++ b/src/modules/user/application/dtos/UserDTO.ts
@@ -0,0 +1,6 @@
+export interface UserDTO {
+  did: string;
+  handle?: string;
+  linkedAt: Date;
+  lastLoginAt: Date;
+}
diff --git a/src/modules/user/application/mappers/UserMap.ts b/src/modules/user/application/mappers/UserMap.ts
new file mode 100644
index 00000000..f203362c
--- /dev/null
+++ b/src/modules/user/application/mappers/UserMap.ts
@@ -0,0 +1,45 @@
+import { User } from "../../domain/User";
+import { UserDTO } from "../dtos/UserDTO";
+import { DID } from "../../domain/value-objects/DID";
+import { Handle } from "../../domain/value-objects/Handle";
+import { UniqueEntityID } from "src/shared/domain/UniqueEntityID";
+
+export class UserMap {
+  public static toDTO(user: User): UserDTO {
+    return {
+      did: user.did.value,
+      handle: user.handle?.value,
+      linkedAt: user.linkedAt,
+      lastLoginAt: user.lastLoginAt
+    };
+  }
+
+  public static toDomain(dto: UserDTO): User {
+    const didOrError = DID.create(dto.did);
+    
+    if (didOrError.isErr()) {
+      throw new Error(`Could not create DID: ${didOrError.error.message}`);
+    }
+
+    let handle: Handle | undefined;
+    if (dto.handle) {
+      const handleOrError = Handle.create(dto.handle);
+      if (handleOrError.isOk()) {
+        handle = handleOrError.value;
+      }
+    }
+
+    const userOrError = User.create({
+      did: didOrError.value,
+      handle,
+      linkedAt: dto.linkedAt,
+      lastLoginAt: dto.lastLoginAt
+    }, new UniqueEntityID(dto.did));
+
+    if (userOrError.isErr()) {
+      throw new Error(`Could not create User: ${userOrError.error.message}`);
+    }
+
+    return userOrError.value;
+  }
+}
diff --git a/src/modules/user/application/services/IOAuthProcessor.ts b/src/modules/user/application/services/IOAuthProcessor.ts
new file mode 100644
index 00000000..4178ae59
--- /dev/null
+++ b/src/modules/user/application/services/IOAuthProcessor.ts
@@ -0,0 +1,12 @@
+import { Result } from "src/shared/core/Result";
+import { OAuthCallbackDTO } from "../dtos/OAuthCallbackDTO";
+
+export interface AuthResult {
+  did: string;
+  handle?: string;
+}
+
+export interface IOAuthProcessor {
+  generateAuthUrl(handle?: string): Promise<Result<string>>;
+  processCallback(params: OAuthCallbackDTO): Promise<Result<AuthResult>>;
+}
diff --git a/src/modules/user/application/services/ITokenService.ts b/src/modules/user/application/services/ITokenService.ts
new file mode 100644
index 00000000..d7967253
--- /dev/null
+++ b/src/modules/user/application/services/ITokenService.ts
@@ -0,0 +1,9 @@
+import { Result } from "src/shared/core/Result";
+import { TokenPair } from "../dtos/TokenDTO";
+
+export interface ITokenService {
+  generateToken(did: string): Promise<Result<TokenPair>>;
+  validateToken(token: string): Promise<Result<string | null>>; // Returns DID if valid
+  refreshToken(refreshToken: string): Promise<Result<TokenPair | null>>;
+  revokeToken(refreshToken: string): Promise<Result<void>>;
+}
diff --git a/src/modules/user/application/use-cases/CompleteOAuthSignInUseCase.ts b/src/modules/user/application/use-cases/CompleteOAuthSignInUseCase.ts
new file mode 100644
index 00000000..b4fa610b
--- /dev/null
+++ b/src/modules/user/application/use-cases/CompleteOAuthSignInUseCase.ts
@@ -0,0 +1,90 @@
+import { UseCase } from "src/shared/core/UseCase";
+import { Result, err, ok } from "src/shared/core/Result";
+import { AppError } from "src/shared/core/AppError";
+import { IOAuthProcessor } from "../services/IOAuthProcessor";
+import { ITokenService } from "../services/ITokenService";
+import { IUserRepository } from "../../domain/repositories/IUserRepository";
+import { OAuthCallbackDTO } from "../dtos/OAuthCallbackDTO";
+import { TokenPair } from "../dtos/TokenDTO";
+import { DID } from "../../domain/value-objects/DID";
+import { Handle } from "../../domain/value-objects/Handle";
+import { User } from "../../domain/User";
+import { CompleteOAuthSignInErrors } from "./errors/CompleteOAuthSignInErrors";
+import { IUserAuthenticationService } from "../../domain/services/IUserAuthenticationService";
+
+export type CompleteOAuthSignInResponse = Result<
+  TokenPair,
+  | CompleteOAuthSignInErrors.InvalidCallbackParamsError
+  | CompleteOAuthSignInErrors.AuthenticationFailedError
+  | CompleteOAuthSignInErrors.TokenGenerationError
+  | AppError.UnexpectedError
+>;
+
+export class CompleteOAuthSignInUseCase
+  implements UseCase<OAuthCallbackDTO, Promise<CompleteOAuthSignInResponse>>
+{
+  constructor(
+    private oauthProcessor: IOAuthProcessor,
+    private tokenService: ITokenService,
+    private userRepository: IUserRepository,
+    private userAuthService: IUserAuthenticationService
+  ) {}
+
+  async execute(request: OAuthCallbackDTO): Promise<CompleteOAuthSignInResponse> {
+    try {
+      // Validate callback parameters
+      if (!request.code || !request.state) {
+        return err(new CompleteOAuthSignInErrors.InvalidCallbackParamsError());
+      }
+
+      // Process OAuth callback
+      const authResult = await this.oauthProcessor.processCallback(request);
+      
+      if (authResult.isErr()) {
+        return err(new CompleteOAuthSignInErrors.AuthenticationFailedError(authResult.error.message));
+      }
+
+      // Create DID value object
+      const didOrError = DID.create(authResult.value.did);
+      if (didOrError.isErr()) {
+        return err(new CompleteOAuthSignInErrors.AuthenticationFailedError(didOrError.error.message));
+      }
+      const did = didOrError.value;
+
+      // Create Handle value object if available
+      let handle: Handle | undefined;
+      if (authResult.value.handle) {
+        const handleOrError = Handle.create(authResult.value.handle);
+        if (handleOrError.isOk()) {
+          handle = handleOrError.value;
+        }
+      }
+
+      // Validate user credentials through domain service
+      const authenticationResult = await this.userAuthService.validateUserCredentials(did, handle);
+      
+      if (authenticationResult.isErr()) {
+        return err(new CompleteOAuthSignInErrors.AuthenticationFailedError(authenticationResult.error.message));
+      }
+
+      const user = authenticationResult.value.user;
+      
+      // Record login
+      user.recordLogin();
+      
+      // Save updated user
+      await this.userRepository.save(user);
+
+      // Generate tokens
+      const tokenResult = await this.tokenService.generateToken(did.value);
+      
+      if (tokenResult.isErr()) {
+        return err(new CompleteOAuthSignInErrors.TokenGenerationError(tokenResult.error.message));
+      }
+
+      return ok(tokenResult.value);
+    } catch (error: any) {
+      return err(new AppError.UnexpectedError(error));
+    }
+  }
+}
diff --git a/src/modules/user/application/use-cases/GetCurrentUserUseCase.ts b/src/modules/user/application/use-cases/GetCurrentUserUseCase.ts
new file mode 100644
index 00000000..f6249f23
--- /dev/null
+++ b/src/modules/user/application/use-cases/GetCurrentUserUseCase.ts
@@ -0,0 +1,52 @@
+import { UseCase } from "src/shared/core/UseCase";
+import { Result, err, ok } from "src/shared/core/Result";
+import { AppError } from "src/shared/core/AppError";
+import { IUserRepository } from "../../domain/repositories/IUserRepository";
+import { UserDTO } from "../dtos/UserDTO";
+import { DID } from "../../domain/value-objects/DID";
+import { GetCurrentUserErrors } from "./errors/GetCurrentUserErrors";
+import { UserMap } from "../mappers/UserMap";
+
+export interface GetCurrentUserDTO {
+  did: string;
+}
+
+export type GetCurrentUserResponse = Result<
+  UserDTO,
+  GetCurrentUserErrors.UserNotFoundError | AppError.UnexpectedError
+>;
+
+export class GetCurrentUserUseCase
+  implements UseCase<GetCurrentUserDTO, Promise<GetCurrentUserResponse>>
+{
+  constructor(private userRepository: IUserRepository) {}
+
+  async execute(request: GetCurrentUserDTO): Promise<GetCurrentUserResponse> {
+    try {
+      const didOrError = DID.create(request.did);
+      
+      if (didOrError.isErr()) {
+        return err(new GetCurrentUserErrors.UserNotFoundError());
+      }
+
+      const userResult = await this.userRepository.findByDID(didOrError.value);
+      
+      if (userResult.isErr()) {
+        return err(new AppError.UnexpectedError(userResult.error));
+      }
+
+      const user = userResult.value;
+      
+      if (!user) {
+        return err(new GetCurrentUserErrors.UserNotFoundError());
+      }
+
+      // Map domain entity to DTO
+      const userDTO = UserMap.toDTO(user);
+      
+      return ok(userDTO);
+    } catch (error: any) {
+      return err(new AppError.UnexpectedError(error));
+    }
+  }
+}
diff --git a/src/modules/user/application/use-cases/InitiateOAuthSignInUseCase.ts b/src/modules/user/application/use-cases/InitiateOAuthSignInUseCase.ts
new file mode 100644
index 00000000..cdeb9b9c
--- /dev/null
+++ b/src/modules/user/application/use-cases/InitiateOAuthSignInUseCase.ts
@@ -0,0 +1,44 @@
+import { UseCase } from "src/shared/core/UseCase";
+import { Result, err, ok } from "src/shared/core/Result";
+import { AppError } from "src/shared/core/AppError";
+import { IOAuthProcessor } from "../services/IOAuthProcessor";
+import { Handle } from "../../domain/value-objects/Handle";
+import { InitiateOAuthSignInErrors } from "./errors/InitiateOAuthSignInErrors";
+
+export interface InitiateOAuthSignInDTO {
+  handle?: string;
+}
+
+export type InitiateOAuthSignInResponse = Result<
+  { authUrl: string },
+  InitiateOAuthSignInErrors.InvalidHandleError | AppError.UnexpectedError
+>;
+
+export class InitiateOAuthSignInUseCase
+  implements UseCase<InitiateOAuthSignInDTO, Promise<InitiateOAuthSignInResponse>>
+{
+  constructor(private oauthProcessor: IOAuthProcessor) {}
+
+  async execute(request: InitiateOAuthSignInDTO): Promise<InitiateOAuthSignInResponse> {
+    try {
+      // Validate handle if provided
+      if (request.handle) {
+        const handleOrError = Handle.create(request.handle);
+        if (handleOrError.isErr()) {
+          return err(new InitiateOAuthSignInErrors.InvalidHandleError(handleOrError.error.message));
+        }
+      }
+
+      // Generate auth URL
+      const authUrlResult = await this.oauthProcessor.generateAuthUrl(request.handle);
+      
+      if (authUrlResult.isErr()) {
+        return err(new AppError.UnexpectedError(authUrlResult.error));
+      }
+
+      return ok({ authUrl: authUrlResult.value });
+    } catch (error: any) {
+      return err(new AppError.UnexpectedError(error));
+    }
+  }
+}
diff --git a/src/modules/user/application/use-cases/RefreshAccessTokenUseCase.ts b/src/modules/user/application/use-cases/RefreshAccessTokenUseCase.ts
new file mode 100644
index 00000000..92fe1735
--- /dev/null
+++ b/src/modules/user/application/use-cases/RefreshAccessTokenUseCase.ts
@@ -0,0 +1,39 @@
+import { UseCase } from "src/shared/core/UseCase";
+import { Result, err, ok } from "src/shared/core/Result";
+import { AppError } from "src/shared/core/AppError";
+import { ITokenService } from "../services/ITokenService";
+import { TokenPair } from "../dtos/TokenDTO";
+import { RefreshAccessTokenErrors } from "./errors/RefreshAccessTokenErrors";
+
+export interface RefreshAccessTokenDTO {
+  refreshToken: string;
+}
+
+export type RefreshAccessTokenResponse = Result<
+  TokenPair,
+  RefreshAccessTokenErrors.InvalidRefreshTokenError | AppError.UnexpectedError
+>;
+
+export class RefreshAccessTokenUseCase
+  implements UseCase<RefreshAccessTokenDTO, Promise<RefreshAccessTokenResponse>>
+{
+  constructor(private tokenService: ITokenService) {}
+
+  async execute(request: RefreshAccessTokenDTO): Promise<RefreshAccessTokenResponse> {
+    try {
+      const tokenResult = await this.tokenService.refreshToken(request.refreshToken);
+      
+      if (tokenResult.isErr()) {
+        return err(new AppError.UnexpectedError(tokenResult.error));
+      }
+
+      if (!tokenResult.value) {
+        return err(new RefreshAccessTokenErrors.InvalidRefreshTokenError());
+      }
+
+      return ok(tokenResult.value);
+    } catch (error: any) {
+      return err(new AppError.UnexpectedError(error));
+    }
+  }
+}
diff --git a/src/modules/user/application/use-cases/errors/CompleteOAuthSignInErrors.ts b/src/modules/user/application/use-cases/errors/CompleteOAuthSignInErrors.ts
new file mode 100644
index 00000000..6dd243e5
--- /dev/null
+++ b/src/modules/user/application/use-cases/errors/CompleteOAuthSignInErrors.ts
@@ -0,0 +1,21 @@
+import { UseCaseError } from "src/shared/core/UseCaseError";
+
+export namespace CompleteOAuthSignInErrors {
+  export class InvalidCallbackParamsError extends UseCaseError {
+    constructor() {
+      super("The OAuth callback parameters are invalid or missing");
+    }
+  }
+
+  export class AuthenticationFailedError extends UseCaseError {
+    constructor(message: string = "Authentication failed") {
+      super(message);
+    }
+  }
+
+  export class TokenGenerationError extends UseCaseError {
+    constructor(message: string = "Failed to generate authentication tokens") {
+      super(message);
+    }
+  }
+}
diff --git a/src/modules/user/application/use-cases/errors/GetCurrentUserErrors.ts b/src/modules/user/application/use-cases/errors/GetCurrentUserErrors.ts
new file mode 100644
index 00000000..52638c58
--- /dev/null
+++ b/src/modules/user/application/use-cases/errors/GetCurrentUserErrors.ts
@@ -0,0 +1,9 @@
+import { UseCaseError } from "src/shared/core/UseCaseError";
+
+export namespace GetCurrentUserErrors {
+  export class UserNotFoundError extends UseCaseError {
+    constructor() {
+      super("User not found");
+    }
+  }
+}
diff --git a/src/modules/user/application/use-cases/errors/InitiateOAuthSignInErrors.ts b/src/modules/user/application/use-cases/errors/InitiateOAuthSignInErrors.ts
new file mode 100644
index 00000000..b36eb185
--- /dev/null
+++ b/src/modules/user/application/use-cases/errors/InitiateOAuthSignInErrors.ts
@@ -0,0 +1,9 @@
+import { UseCaseError } from "src/shared/core/UseCaseError";
+
+export namespace InitiateOAuthSignInErrors {
+  export class InvalidHandleError extends UseCaseError {
+    constructor(message: string) {
+      super(`Invalid handle format: ${message}`);
+    }
+  }
+}
diff --git a/src/modules/user/application/use-cases/errors/RefreshAccessTokenErrors.ts b/src/modules/user/application/use-cases/errors/RefreshAccessTokenErrors.ts
new file mode 100644
index 00000000..3f5dea49
--- /dev/null
+++ b/src/modules/user/application/use-cases/errors/RefreshAccessTokenErrors.ts
@@ -0,0 +1,9 @@
+import { UseCaseError } from "src/shared/core/UseCaseError";
+
+export namespace RefreshAccessTokenErrors {
+  export class InvalidRefreshTokenError extends UseCaseError {
+    constructor() {
+      super("Invalid or expired refresh token");
+    }
+  }
+}
diff --git a/src/modules/user/application/use-cases/index.ts b/src/modules/user/application/use-cases/index.ts
new file mode 100644
index 00000000..51b2a844
--- /dev/null
+++ b/src/modules/user/application/use-cases/index.ts
@@ -0,0 +1,4 @@
+export * from './InitiateOAuthSignInUseCase';
+export * from './CompleteOAuthSignInUseCase';
+export * from './GetCurrentUserUseCase';
+export * from './RefreshAccessTokenUseCase';