As a Leeds fan, I feel able to comment on MOS entirely as a neutral as we clearly have no credible contenders this year.
The likely candidates for me are Sam Tomkins, Michael Monaghan, Rangi Chase, James Roby.
For me it depends what you're looking for. All 4 have been hugely influential for their teams this year. I would personally eliminate Chase as with him and Tomkins you're largely looking at their attacking threat, and Tomkins is the most effective attacking player in the league.
Stats can tell fibs, but Roby's are certainly impressive and he's been really good whenever I've watched Saints this year. Monaghan has been more quietly efficient, but he's been the pivot when Wire have been at their best and they don't look anything like the same team when he's not playing to me.
Any of the 3 of them would be worthy candidates. Roby already has one MOS he didn'tr really deserve though, so it wouldn't break my heart if he missed out this year.
On the O'Loughlin debate, I think his trouble (if it is a trouble) is that he's the type of player who influences a game in more subtle ways, including his command of the defensive line. I like to think that I watch more critically than most, but even I don't tend to notice him much when he's in opposition. It's when he's in an England/GB shirt and thus "on my side" that he appears more prominently. I haven't always rated him that highly because some of his best attributes are relatively easily missed if you're not watching for them.
He might get a coaches' player of the year, but probably not a players or fans award. That's the nature of what is, effectively a popularity contest. I'm sure if he keeps getting to wave shiny silver trophies around at the business end of the season he'll probably be able to live with it....