Pct = 1.3465*OPSDIFF + .5
That is based on a regression done on all teams from 2010-2014. I used team averages over the period for both pct and OPSDIFF.
The table below shows each team's OPS so far this season along with their OPS allowed (OPSA), their actual winning pct and then the differential in their win total from what the formula would predict.
The Orioles have a large negative OPS differential yet are still close to a .500 team at .488. So they have won about 9 more games than expected. I will have some discussion below about the Orioles as well as the Twins and Yankees (who have won about 8 fewer games than expected). Also, at the end of this post is a table with each team's actual win and loss totals through Monday.
Orioles: Their run differential is -75, so I thought maybe they had a great record in 1-run games. It is good at 12-8, but I don't think that accounts for winning 9 extra games. With runners in scoring position, their OPS is .856 while just .711 with none on. So far this year, for all of MLB, those numbers are .771 and .734. Maybe that has helped the Orioles score more runs than expected. But maybe not.
Over the years 2010 - 2012 here is the regression generated team runs per game based on OBP and SLG
R/G = 14.71*OBP + 10.37*SLG - 4.57
The O's have an overall OBP of .308 and SLG of .422. The equation estimates that they would score 4.34 runs per game while it was actually 4.44. Not a big difference.
Their pitching might explain things. They have allowed an .830 OPS with none on but just .786 with RISP. So that is .044 better, combined with the .037 differential for all of MLB in the opposite direction and we have a .081 swing. So that could explain quite a bit. The O's pitchers are just doing well with RISP.
But the O's pitchers have allowed OBP and SLG of .355 & .466. That estimates to 5.48 runs per game while the actual is 5.34. Combine that with the extra 0.10 from offense and we have a swing of .24 runs per game. Over 82 games, that is about 20 runs or just 2 extra wins.
Hard to see what is going on. I would have expected a much better record in 1-run games. If we give them to extra wins there, we are still only at 4 extra wins far below the 9 we get. But, their overall predicted pct is .377. Over 20 1-run games, that would be 7.5 wins, or 4.5 less than they actually have, accounting for half the difference.
So I looked at their OPS and OPSA in close and late situations. They are .698 and .710, respectively. I thought they might have a big positive differential here, but they don't.
The O's hitters have in High, Medium & Low leverage cases their hitters have an OPS of .752, .783 and .682. Their pitchers have .741, .783 and .883. So the positive differential in high cases probably helps. See comments on Twins.
Twins: They are 9-4 in 1-run games and they have a -55 run differential. So maybe they have 2-3 more wins than expected in 1-run games but that is far below the extra 7 wins. Their pitchers and hitters don't do any better than they normally do with runners on or with RISP (alot worse in some cases). They have a .633 OPS win close and late situations while they give up .676. Not sure how they are doing so well in 1-run games.
They do allow a only a .689 OPS in high leverage situations while it is .841 & .803 in medium and low cases. So maybe when it is close and late they do really well with runners on. In High, Medium & Low cases, their hitters have .761, .746 and .730. So that means they have a very good differential in high leverage cases.
Yankees: They are 9-16 in 1-run games. This probably explains alot of why they have won 8 fewer games than expected. In High, Medium & Low leverage cases their hitters have an OPS of .716, .822 and .825. Their pitchers have .725, .717 and .672. So the negative differential in high cases probably hurts.
But I do have a regression equation that uses the OPS differentials in the High, Medium and Low cases. Here it is
Pct = .5 + .306*LOW +.420*MED + .564*HIGH
Using this equation, Yankees have won only 3.4 fewer games than expected. The Orioles and Twins have won 3.53 and 2.77 more games than expected, respectively. Only one team ends up more than 5 wins off, the A's at -5.31. So taking into account how teams do based on leverage makes a big difference. Maybe differences still exist because of errors, turning DPs and/or base running. Also, I used OBP & SLG instead of OPS, things might still get more accurate.