Clarification:
Ok, most if not every DO school will require you to pass COMLEX I,II in order to graduate.Period! Now, if you complete Steps 1 to 3 these exams are all that are required of you (a DO) in every state to obtain medical licensure.
Residency programs will require you to get licensed during your residency.
You must have licensure, whether it be temporary or provisional to even complete your residency in any field , in any state after your Intern year at least (Oklahoma requires passage of all three exams to obtain licensure, which is required of DOs after completion of an intern year.)
Residency competitiveness:
The issue here is this:MD program directors have said it time and again, it makes DOs easier to compare to the MD applicants. If it makes things easier for them, doesn't it follow that it might increase your odds of matching at these places?
I have not heard much discussion on whether USMLE Step 2 made any difference, but I was told flat to my face by multiple MD program directors
"You will be glad you took USMLE Step 1-and by the way, you have a respectable score." (One of these guys was at the program I matched at! Imagine that!)
Hey, it should be an honor to be compared with MDs on the sole basis of USMLE grades--not the initials behind your name, thank God! Seriously.
So this is what DO students do: Since usually all they need for licensure by the time they finish is COMLEX Step III, they opt to take this instead of three more exams (USMLE 1-3) that aren't even required anymore to obtain US medical licensure. Again, COMLEX 1-3 is sufficient to obtain licensure in every state. So, during the intern year often, you take the COMLEX Step 3 for your license, and you are done.
During the interview season, or more commonly at the same time they take their COMLEX step one, a lot of DO students will take/pass the USMLE Step 1 soley to increase their competitiveness as described above. Most never take the second or third steps simply because: they are superfluous (sp?), they cost extra money, and are these enough reasons?
All this being said, there are definite speciaties in the MD world that taking even USMLE step 1 is superfluous: These are:
Family Medicine, Internal Medicine (less competitive locations), Pathology, some OBGYN, Psychiatry to name a few.
I would strongly recommend taking USMLE Step 1 for:
Anesthesiology, Surgery, Radiology, Dermatology, PM&R etc
Hope this helps!