We're a little late getting to this, which is sad because you don't sign players like Adam Oates every day.
Like many of the Senators' college free agent signings so far, Da Costa has demonstrated flashes of excellence amid stretches of mediocrity. With 13 forwards already signed to one-way deals, it was unlikely he'd have a shot at the NHL roster anyway. The deal gives him an opportunity to continue to develop in Binghamton, where he had 36 points (13G, 23A) in 46 games last year.
This move is the right one for both sides, as Ottawa gets to hold on and evaluate an asset they've been developing, and Da Costa gets a chance to prove his worth to the team with some better linemates.
And hopefully, he can afford to buy a helmet that fits now.