If you only want to learn about a single fairly small concept like Big O, youtube is great - https://www.youtube.com/watch?v=Q_1M2JaijjQ. (good explanation, longer than a few minutes, and a great channel in general)

If you want to get deeper (or wider?), you can get a textbook (e.g. Introduction to Algorithms by Cormen) or a university course like MIT 6.006 but obviously those will contain a lot more than just this topic.