Can you go straight into an Army oral surgery residency after dental school if on the HPSP scholarship?
yes, it is rare, but possible. i wouldn't count on it.
first, before d school, you must have attended OBLC;
second, while in d school, you must have a very high gpa (somewhere in top 5-10%), and be selected in the program; AND
third, while you're waiting for it to start in 2 years, suddenly a spot opens up b/c the person in line who starts in 1 year drops out, or the army for some reason decides to increase the class size.
as you can see, it can be done, but i wouldn't count on it.
I've heard people say that you have to serve as a gp for some time first. Is this true?
yes, at least a year as a gp, b/c the army selects candidates at least 2 years in advance.
Also, how competitive is it to get into an Army OMS residency vs a private one.
private: yeah, very, very competitive
army: yeah, perhaps not as bad, but still very competitive
Will the 4-yr thing vs 6-yr thing cause any future problems to $ flow?
if you have needed a procedure done on you, and you need to select one doctor--one surgeon with dds vs one with dds AND md, and they charge the same... well, go figure.
hope it helps.