USMLE has 3 steps....I, II, and III
step one is taken after your first two year. Step two is taken upon graduation, and Step three is taken after your internship year.
Osteopathic students do not have to take the USMLE, but some allopathic residencies might like to see you take it so it will be easier for them to compare you to other students.
I've heard that studying for and taking the COMLEX prepares you for the USMLE, so it is mostly just going through the hassle and headache of taking another exam.