IMHO, it seems clear that OOTP is designed so that any player can play any position for a few innings without a major problem. I suspect that is because the AI often ends up creating fielding assignment problems toward the end of a game. If an OF couldn't function as a catcher, for example, the game would break down from time to time. This is not a good thing, but may be expedient, at least until the causes of these fielding assignment problems can be fixed.
OTOH, Kelric is correct about the long-term. I tried this myself: playing everyone on a team out of position for a season. The results were pretty grotesque.
