OTIS is a training program run by Evan Chen for math olympiad training. This document has all the related information.

I am one of the instructors of the program, and I take 3 hour one-on-one classes for a small group of students as a part of the program. If you're interested in this, you can contact me but please note that I usually won't be able to take additional students due to the workload. That being said, I am open to providing help in whatever way possible :)