Adding OAuth login usually means letting users sign in with an external identity provider like Google, GitHub, Microsoft, or Facebook.

Here’s the typical approach:

1) Pick an OAuth/OpenID provider

Common choices:

• Google
• GitHub
• Microsoft
• Apple
• Auth0 / Okta / Firebase Auth (hosted identity platforms)

If you’re doing “login,” you usually want OpenID Connect (OIDC) on top of OAuth 2.0, because OAuth alone is for authorization, while OIDC gives you identity/login.

2) Register your app with the provider

You’ll get:

• Client ID
• Client Secret (for backend apps)
• Redirect URI / Callback URL

Example redirect URI:

• https://yourapp.com/auth/callback
• or http://localhost:3000/auth/callback for local dev

3) Use the Authorization Code flow

This is the standard secure flow for web/mobile apps:

1. User clicks “Sign in with Google”
2. Your app redirects them to provider’s authorization URL
3. User logs in and consents
4. Provider redirects back to your callback URL with a code
5. Your backend exchanges the code for tokens
6. You use the user info / ID token to create a session in your app

4) Store users in your database

Typically save:

• provider name
• provider user ID
• email
• name
• avatar
• local app user ID

This lets you link external accounts to your own user system.

5) Create a session or JWT

After successful login, your app should issue:

• a server session cookie, or
• a JWT access token

For most web apps, secure HTTP-only cookies are a good choice.

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Example flow

Frontend button

<a href="/auth/google">Sign in with Google</a>

Backend route

Redirect user to provider:

app.get('/auth/google', (req, res) => {
  const url = buildGoogleAuthUrl();
  res.redirect(url);
});

Callback route

Handle provider response:

app.get('/auth/callback', async (req, res) => {
  const { code } = req.query;

  const tokens = await exchangeCodeForTokens(code);
  const userInfo = await getUserInfo(tokens.access_token);

  const user = await findOrCreateUser(userInfo);
  createSession(res, user);

  res.redirect('/dashboard');
});

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Security tips

• Always use HTTPS
• Use state parameter to prevent CSRF
• Use PKCE for public clients like mobile apps/SPAs
• Don’t expose client secret in frontend code
• Verify ID tokens properly
• Restrict redirect URIs exactly

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Best practices by app type

Web app

• Backend handles OAuth code exchange
• Use server session cookies

SPA

• Use Authorization Code + PKCE
• Avoid storing long-lived tokens in localStorage if possible

Mobile app

• Use system browser / app auth flow
• Use PKCE
• Deep link/callback handling

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Easiest implementation options

If you want to avoid building OAuth manually, use:

• NextAuth.js for Next.js apps
• Passport.js for Node.js
• Auth0
• Firebase Authentication
• Supabase Auth
• Clerk

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If you want, I can show you:

1. how to do this in Node/Express,
2. React + backend,
3. Next.js, or
4. a specific provider like Google login.