If Santander plays left and Stewart plays right Mountcastle will never play the outfield. Mountcastle's arm is not good enough to play right and he is not good enough to move Santander out of left.
So Mountcastle better hope that Stewart is starting the season in left field.
As for Mountcastle and Stewart, Mountcastle is going to be playing just about everyday between LF, DH and 1B once the Orioles get the extra year of control (I believe somewhere around 7-10 games). Stewart will get his ABs in LF, Rf and DH as well, but I don't see him as a priority play.
If Santander will be a go in RF, the outfield will typically be Mountcastle, Hays and Santander with Stewart being the 4th OF with Santander moving to CF to give Hays a break against maybe a tough righty.
I expect this season to play almost like a 60-game spring training for the Orioles with a lot of moving parts and essentially want amounts to try outs.
I think there are a lot of variables for where Mountcastle could get his ABs. If the goal is primarily LF for him that could come at the expense of Stewart, but also Nunez, Riuz, Davis, or Santander. Given the short Spring Training 2.0, there's also an increased chance of muscle related injuries which could open the door, particularly if it's one of the aforementioned players.
In this season, I want to see a lot of Hays and Mountcastle to get an idea of what we have. I want to see if Santander can build off last year; if Stewart can prove to be a useful big leaguer. I want to see if Davis can build off what looked like a nice start in Spring Training 1.0 or if this is his last chance. I want to see Nunez get reps defensively somewhere. I like his bat, but not sure I like it enough as a DH-only to warrant a spot when more players begin graduating or when Mancini returns next year. Of course, all of that is just on the offensive/position player side. I would like to get looks at a lot of pitchers as well. This should be a fun year to see what might be some guys last chance to prove themselves (though with no minor league season, that window for a few guys may be extended a bit with development stalled).