If he is, then it's exactly what we need, a new pitbull!
I can't really judge whether this is a good signing or not because I can't recall having seen the lad play before, but Neal Ardley's assessment of what we've been missing sounds spot on and if Jim O'Brien is capable of giving us that, then we should improve. He's certainly played enough games in his career to suggest he's been a first-team level player wherever he's gone.
Plus, if you're looking to add a bit more aggression to your team, you generally can't go wrong signing a Scot. As Bill Shankly said:
"If you’ve got three Scots in your side, you’ve got a chance of winning something. If you’ve got any more, you’re in trouble!" 