The crux of OAuth is an access token, which is sort of like an API key for a specific user. An app gets an access token, and then they can use that to take actions on the user's behalf, or access data for a user.

Big O notation is a way of describing the performance of a function without using time. Rather than timing a function from start to finish, big O describes how the time grows as the input size increases. It is used to help understand how programs will perform across a range of inputs.