It depends on where you are hoping to do residency... By and large, psychiatry is a buyers market and you won't need to take the USMLE (I am also a DO and I chose not to take it). Some of the most competitive programs or places that aren't as familiar with DO's might raise eyebrows or require that you take USMLE, but I didn't find it to be a problem when I applied and interviewed. If you already have a few places in mind (or a general location where you think you might want to be), you can always make a few quick phone calls and just ask if they accept COMLEX scores or if you have to take USMLE...