Sometimes if you see something for yourself, its better than a million words
I would also not recommend adding Oscar while arowana is so small
I did it twice in the past, it did not work
2 months ago my son got a chilli red about same size as yours and he has 2 small oscars about 10cm and again we had to move the oscars for now
Oscars are everywhere but very much at the top especially when you feed
Oscarsare very greedy and dont know when to stop eating and they will take the arowana's food even if they have to spit out what is in their mouth just so they can have the new food
They also very fast at catching the food
Personally, i would wait until the arowana is eating well and swims fast for her food before you add the oscar.
If you already have the oscar, watch carefully if your arowana is getting enough food
Good luck with your baby