What, precisely, do you mean by "officer?" It seems like a pedantic question, but it's really not. The army contained ranks like the centurions, who might be considered something like junior officers, but the "commanders," so to speak, of the army were not career soldiers but magistrates. Prior to the Principate very few equites would be considered for centurions and other such ranks, but would instead hold the military tribunate. But I'm not sure if that's what you're really asking about, as the military tribunate was really a political magistracy and not a career position