I personally wouldn't recommend the Oscar and a Flowerhorn together, that would almost surely end in disaster unless in a huge community tank. Some may disagree but imo Flowerhorns are best kept solo.
The pleco is kind of 50/50, depends on character (and size) of your FH. It helps if the pleco is already in the tank first before adding the FH). You'd just have to watch it closely and see how it worked out. However if the FH were aggressive toward it I'd soon separate them -- if it happened to eat the pleco it would be a goner too.