Agreed with what is posted above.
For DO students, COMLEX 1, and both parts of COMLEX 2 are graduation requirement for all DO schools. Once you graduate, you only need to take COMLEX 3 (1 day exam) and you completed the COMLEX series and thus the testing requirement for licensure in all 50 states + PR + DC.
DO students take USMLE mainly for residency applications.
If you want to take USMLE for licensure, you will have to take all 3 USMLE Steps (Step 1, Step 2-CS, Step 2-CK, Step 3) in order to be eligible for licensure via USMLE (and there are 6-7 states that will NOT recognize your USMLE for licensure, they require COMLEX).
So taking USMLE for licensure purpose means more $$$, more time (since you have to take 3 out of 4 required COMLEX tests anyway to graduate), and not finishing COMLEX series will prevent you from practicing in certain states (ie California, Florida, Pennsylvania, etc)
As for board certification - that's the "specialty board" that people take at the end of residency (or fellowship). Please don't refer to USMLE or COMLEX as "board certification".