OAuth 2.0 in Plain English (Flows Explained Without Confusion)

OAuth 2.0 is everywhere.

You use it every time you:

• sign in with Google
• connect an app to your GitHub account
• authorize a payment system
• grant access to a third-party tool

But OAuth documentation is famously confusing — full of jargon like "authorization servers," "resource owners," and "clients."

Let's drop the jargon and explain OAuth like you're hearing it for the first time.

⭐ What Problem Does OAuth Actually Solve?

Imagine you want App A to access data from App B without giving App A your password.

Example:

You want Notion (App A) to access your Google Calendar (App B).

• You don't want to give Notion your Google password.
• You do want Notion to read/write to your calendar.

OAuth makes this secure and possible.

OAuth lets you give apps limited access to your data — without sharing your password.

⭐ The Real Players (But in Normal Language)

Forget the official names. Here's what they really mean:

That's it. Four players — one of which is you.

⭐ Step-by-Step: What Actually Happens During OAuth

Let's use the example:

➡️ Notion wants permission to access your Google Calendar.

Here's what happens behind the scenes:

Step 1 — Notion sends you to Google to log in

You click: "Connect Google Calendar"

Notion does not collect your Google password. Instead, it redirects you to Google's login page.

Step 2 — Google asks: "Do you allow this?"

You see a page that says:

"Notion wants to view your calendar. Allow or Deny?"

This is Google's authorization server verifying whether YOU want this connection.

Step 3 — You click "Allow"

Google now issues Notion a one-time authorization code. This code is short-lived and useless if stolen.

Step 4 — Notion exchanges the code for an access token

Notion sends the code to Google (secretly, from the backend):

Here's the authorization code.
Please give me an access token.

Google replies:

Here's your access token.
You can now access the user's calendar.

Step 5 — Notion uses the access token to call the calendar API

This token works like a temporary key.

• It does NOT reveal your password
• It expires
• It has limited permissions
• It may only allow read-only access

Final Step — Access token expires → Notion uses a refresh token

To keep you logged in silently:

• the access token expires quickly
• the refresh token lasts longer and can request new access tokens

This keeps everything secure.

⭐ The OAuth Flows Explained Simply

OAuth has multiple "flows." These are just different ways to perform the same basic idea depending on the app.

Here they are in plain English:

⭐ 1. Authorization Code Flow (Most Popular)

Used by:

• SaaS products
• Web apps
• Backend apps
• Anything with a server

Why? Most secure. Tokens are exchanged on the backend.

Simple definition: Redirect user → ask permission → get a code → exchange code for token.

⭐ 2. PKCE Flow (For Mobile & Single-Page Apps)

Pronounced "pixie."

Used by:

• React apps
• Mobile apps
• Any frontend without a backend

Why? These apps can't safely store secrets. PKCE protects the code exchange.

Simple definition: Authorization Code Flow but with extra steps to prevent interception.

⭐ 3. Client Credentials Flow (Machine → Machine)

Used by:

• microservices
• servers calling other servers
• No user involved

Simple definition: The app logs in as itself using its own credentials.

⭐ 4. Device Code Flow (For TVs, PlayStations, etc.)

Used when you:

• log in on your TV
• see a code
• go to a website on your phone
• authorize the device

Simple definition: Device shows a code → you approve from another device.

⭐ 5. Refresh Token Flow

Not truly a flow — more like a system.

Used when:

• access token expires
• app requests a new one without asking you to log in again

Simple definition: Silent re-authentication.

⭐ Why OAuth Exists (In One Sentence)

OAuth lets apps access your data safely, without sharing your password, and with limited permissions.

That's it.

⭐ The Most Common Misunderstanding

Many people think OAuth = login.

It's not.

• Logging in → Authentication
• Allowing access → Authorization

OAuth handles authorization. Hence the name: Open Authorization.

⭐ When Should You Use OAuth in Your Product?

Choose OAuth if:

• ✔ Your app connects to Google, GitHub, Slack, Notion, Stripe, etc.
• ✔ You need to access user data from another platform
• ✔ You want to avoid storing passwords
• ✔ You want granular permissions
• ✔ You want security best practices
• ✔ You want scalable authentication/authorization

Skip OAuth if:

• ❌ You're building a small internal tool
• ❌ You just need a username/password
• ❌ You don't integrate with third-party services

⭐ Final Thoughts

OAuth is powerful — but the explanations often make it sound more complicated than necessary.

At its core, OAuth is just:

• sending users to a trusted login page
• getting permission
• receiving a safe, limited token
• using that token to access an API

If you understand the why, the how becomes simple.