Clocking in at 21 on our list is another full-time roster player who, due to the lunchpail-ish nature of his game, may be lower on this list than he deserves. Jim O'Brien, ranked 25 on this list last year, surprised a a lot of folks by working his way onto the Senators' lineup for the final third of last season, and he's kept surprising people by fending off numerous up-and-comers to retain that spot on the roster.
Admittedly, Jimbo isn't the flashiest player, and his skating isn't pretty. But it does the job, and it's earned him so much trust from head coach Paul MacLean that O'Brien played in every single playoff game last season, no small feat for a rookie to accomplish.
O'Brien earned and has kept the trust of MacLean by playing the proverbial "200-foot game" and doing exactly what the coach asks him to do. He's become a defensively responsible energy guy who's a vital component of Ottawa's penalty kill; he's tops among forwards in SH TOI/GP this season, was seventh during last year's playoffs, and very quickly climbed to fifth during the 2011-12 season. His production was on the low side last season, but that's not really what he's asked to provide; as long as he plays the defensive side of the game, MacLean will use the team's more offensively gifted forwards to score the goals.
Making a long-term projection for O'Brien is a bit tough, but the fact that he was signed to a two-year extension in the off-season shows that the team sees a very real place for him in the near future. It's highly unlikely that he'll surpass Zack Smith on the depth chart (let alone Kyle Turris or Jason Spezza), but aside from perhaps Derek Grant there aren't really many competitors for his fourth-line centre spot (although Peter Regin has already usurped him there, at least for the time being). O'Brien's not an irreplaceable player, but if he can continue doing what he's doing to stay in MacLean's good books he'll do himself a lot of favours in terms of extending his contract.
And if O'Brien can add to his current status as solid fourth-liner and penalty killing specialist--by becoming, for instance, a highly reliable face-off man or by increasing his physical edge without taking penalties--it'd improve his odds of earning another one-way NHL contract a couple years from now.