The problem: login without sharing your password
Previously, when an app wanted access to your Gmail contacts, you had to give it your Gmail password. Knowing your password = access to all your emails.
OAuth solves this: you don't give the app your password. Instead, you tell Google, "grant this app permission to see my contacts."
Three players
Resource Owner: You, the user.
Authorization Server: The institution holding the data (Google).
Client App: The application you're authorizing.
Flow: the app redirects to Google → Google asks for your approval → you approve → Google gives the app a token → the app uses this token for limited access.
OAuth at Onremo
Onremo supports the OAuth 2.0 standard. From Account → Developer → OAuth applications, you can get a client_id and add a "Sign in with Onremo" button to your own application.
Details: API access guide.