I suppose I see Oviedo as more of a utility man and I think Martinez does too. Martinez has been looking to sign left backs so I think he views Oviedo as not an automatic left back starter. I also think he will look to play Baines in midfield. With the way we play our fullbacks needs a top engine, which Baines still has, but if it starts to wane a little he might be deployed in other roles.
The problem though is Baines is a left back who provides width. Playing him as a DM doesn't work, it's not what he does, also he at the wrong end of his career to suddenly become a tricksy playmaker/forward to fit in the three behind Rom. The only place he fits in the system is at left back.
Brian can play on the left of that three just behind Rom. This is how Bobby has been fitting him in to get him the game time to get him fit again and keep him happy whilst also playing Baines. Unfortunately it's Kev's position who until recently has been the only one of those three behind Rom showing some sort of gumption (maybe not the case after after the summer but who knows)
Garbutt you could argue could play both positions (though his crossing would be lost in our narrow midfield) but he's behind Baines and Brian for the LB slot and Kev and Brian (and perhaps others) for the midfield slot.
There doesn't seem to be many options for Luke, if Brian wasn't there he be up the pecking order in two positions.
The best overall answer might be to cash in on Baines. If we could get some serious money for him this summer then you'd have to go looking for good reasons to keep him and letting one of the other younger two go.