I took both ACT and SAT, but college requires ACT "OR" SAT, can I choose to submit one or the other?

Likely, yes. What are your scores? Pick your best one.

Some colleges require all scores of a certain test (SAT or ACT) but I don’t think any require both. What does school website say?

If it says OR, send the best one. They won’t ever know you took the other one if you don’t send it anyways

Here’s a graph that might help you decide which score to submit. I think there are tables available from other sources also. Google “SAT ACT comparison”

http://www.■■■■■■■■■■■■■■■/comparing-act-and-new-sat-scores/

There are two colleges, Stanford and Georgetown, that require you to send both ACT and SAT scores if you have taken both tests. If applying anywhere else, you can choose to send SAT or, alternatively, ACT.