I think you can put this problem to bed now, pretty much solved. Your parameters were fine so the only other reason that your oscar was sulking and looked a bit beat up was probably aggression. I suspect that although you didn't witness it something went on whilst you were away, or in the evenings maybe. Moving him has taken him away from the aggressor, hence your oscar's perked up. So I wouldn't "debate" on anything, i'd go for a personal tank just for the oscar. Keep your eye on the other tank too. The bully, whichever fish it was, may start bullying another tank mate.
Glad things have worked out.
