I am in your shoes as well. I have a 6ft 125 with a 10in Oscar. I am as well trying to find a tankmates or two for her.
So far she has some Stripped Silver Dollars. She tried to eat one when first introduced, but it was too wide and she couldn't eat it so has learned to leave them alone save a half hearted chase every now and then.
Next we have an p. Ornatus catfish. Same as the SDs, when first introduced she tried to get a taste of some catfish. But Catness Everwisker is quick like lightning and she leavers it alone now.
The Oscar is nearly a year old now. Over the the last year at different times I tried to introduce five different Oscars. She has rejected them all, even killing one. R.I.P. Juan Carlos.
At the moment I do have a Red Terror, and a Green Terror in with her. Both are smaller than her (Oscar is 10in RT,GT are in the 4 to 6in range).
So far she has accepted them, there was a small bite to each of the new comers upon introduction saying "this is my house, but I like you so you can stay"
Who knows if it will stay all happy times. I did learn a valuable thing if your going to try and mix, stay all south or all central, mixing rarely works. Try and get all males, a female dropping eggs randomly adds extra chaos to the tank (learned this the hard way). Lastly a big key to adding with Oscars, make sure the fish is bigger than the O's mouth and will stay that way. I started with Roseline sharks as dithers, she out grew them and ended eating one.