I'll be honest, when it comes to DOs, if they only have a COMPLEX score, they are not typically offered an interview at our program (which isn't Neonatalogy, so take that with a grain of salt). For DOs to be competitive, they need a USMLE score. This isn't meant to be prejudice against DOs, but on paper, board scores are one of the few metrics that are comparable among applicants. If 99% of all the applicants take the USMLE and 1% only take the COMPLEX, it makes it very hard to objectively compare them to the other applicants. If you can't be easily compared to the other applicants, of which there may be 80 for 3-4 spots, your application isn't really considered. Now saying that, there are many programs out there for fellowship training and I'm sure they don't all have the same requirements, but I will say that in my limited experience at 2 different programs (again, not Neo), you need to have a USMLE to get your foot in the door for an interview. It doesn't mean you shouldn't try with what you have, but the reality is the more you can do to level the playing field, the more advantageous it is for you.