I have 4 jacks in one tank, and heck they will turn on each other. I can't imagine the guppy or turtle being there... Both Oscar and Jack will kill both of em eventually. IMO 55 gal is large enough for both jack and Oscar for now. Both are still small. But if your jack is that aggressive I would remove the Oscar and get him another Jack to befriend. I feel really bad for the Oscar. When my jacks are spawning, they scare my other 2 to where they almost jump out the tank! I take em out til the process is over... I remove the fry and they let them come back with no problems. However they do still fight occasionally.
I was told if the Oscar was bigger like yours is it could live with the jacks... Apparently not in your case. Sorry.