>>280636254Technically in in universe time, Osamu's been with Border for months. I don't think we have a fix on his exact join date, but if he got in back in May after Rinji vanished, he's been with Border for 9 months, only two of which he's spent at Tamakoma branch with some decent training. If he wasn't in time to enlist right then, he probably would have had to have waited until September for enlistment, and he's only been with Border for 5 months, two of which were spent at Tamakoma.
But since his focus is split between captaining his squad and improving his own skills, his own improvement suffers. Or to put it another way, he didn't have the chance to spend enough time as a C-rank getting the basics down, so now he's stuck trying to simultaneously get the basics down, get the intermediate skills down, and devise strategy for his squad; all in a time crunch to qualify for the next away mission so that Replica and Yuma can reunite without risking Yuma simply dying before then.
What hurts is that his strategy, Yuma's skill, and Chika's power is just enough to skip them past opponents more on his level, and bounce off the top of B-rank.