I was pretty confident I wanted to match academic IM and then subspecialize when I came in and since I knew as a DO matching at a strong academic IM program would be very difficult I hit the ground running since day 1. My advice is to expose yourself (shadowing, research, finding mentors etc) as much as possible to competitive specialties if you feel like you may be interested in them as early as possible so you can start preparing an app for it. With Step 1 going pass/fail research matters more so hopping on Derm or surgical sub research early on in med school will be muy importante for your app for residency. I dont recommend waiting until you get ur 260-270+ Step 2 score back and saying at the end of M3 hmm i think i want to do Derm or ENT now and all you have to your app is a nice juicy Step 2 score. For these specialties you need more than just a high board score. Find mentors early, cold call/email research coordinators at academic programs, reach out to residents at programs you feel like you may be interested in early and ask for their advice they may even know an academic faculty mentor that they may be able to set you up with (this is what happened to me and led to an academic letter of reccomendation and research at a nice mid tier uni program which helped bolster my app for academic IM). Network network network. This advice can be helpful for anyone but particularly geared toward DOs going for competitive specialties and by no means is a requirement but can really help you down the line