Agree that the Oscar may just be in a transition stage due to the new home. If you decide you want some "friends" for him...you could throw in some "dither" fish. Tiger-barbs or Tin-Foil barbs.
FYI...I do have an Oscar and a Green-Terror in a 75...and they get along great. But with agressive fish...you never know with individual personalities.
I actually had my pair of calico convicts with him and was doing good till I moved them. So I honestly think its cause he's used to having tank mates. I've read in the past that oscars get depressed if they are raised with tank mates and then moved alone.
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