Open Authorization - standard for secure API access without sharing passwords. Like giving a valet key that only works for parking, not accessing your glove compartment.
When you log into Spotify using your Facebook account, OAuth allows Spotify access without seeing your Facebook password.
OAuth is an open standard (not a single cloud service). These managed identity services can act as OAuth 2.0 authorization servers and/or integrate with external OAuth providers to issue and validate access tokens for APIs and apps.