OAuth (Access Token) Vs API Key

You need OAuth only when you want to enable a user of your service to allow a third-party client application to access his/her data hosted in your service without revealing his/her credentials (ID & password) to the application.

What a pair of API key & API secret can do is just authentication of a client application. If it is okay for you to allow an authenticated client application to access a user's data without explicit consent by the user, you don't have to use OAuth.

API keys are for projects/services while token is used to authenticate users. Came across a very good explanation. when and why to use API key