Tom, i'm reading the situation that the OP's nitrates are typically ok. They were only extremely elevated when he'd been away for two weeks and obviously no water changes had been done in that time. That, to me anyway, and how the OP has worded it, suggests the fish have never been exposed to constantly high nitrate. Could be wrong.The Oscar looks stunted it should be bigger than what it is after 5 years. The high Nitrate level caused this and what the Oscar is going through now. The water changes were probably not large enough through those years. The other Cichlids don't show signs of health problems due to them being hardy enough to tolerate the high Nitrate.
d3adlydesignz can you confirm that your water change schedule is good so we can rule out long term nitrate exposure as being a possible component of your oscars problem?
As soon as I saw the tattered fins I thought "aggression" could be part of the problem, either that or the oscar is simply old. But he ruled out aggression, and old age, I reckon the oscars about 6 or 7 maybe, mid life for an Oscar??
That said, the OP was away for two weeks so even he himself can't fully rule out any aggression as he simply hasn't been there to witness any aggro. duanes , if it is aggression related and the aggressor "won" the duel between them, could it be the oscars submissive cowering at the bottom of the tank means the aggressor will leave the oscar alone, for now anyway. As soon as the Oscar gets his confidence back and becomes active will it happen again?
Also, i realise it's only a part shot of his tank but I don't see anything that could act as a hiding place or something to break line of sight.