At what age/size do Oscars start to pair off at?
At what point should I remove an Oscar if it getting chased?

This is the sort of question that needs to be answered by quite a few of us giving our own experiences. It's very difficult to say exactly when Oscars pair off if you've only got two of them and they are hanging around with each other from a very young age.Oscars become sexually mature at approximately 14 months, maybe a little less. However, they may lip lock at a much earlier age that this isn't necessarily anything to do with breeding, it may be one Oscar trying to get a hierarchy over the other.

They can be very rough with each other and it really would be up to your discretion to decide whether this is pure aggression, or breeding ritual. I know for sure that Nicki has got this going on at this very moment with some of her Fish

As OFL has said fish can pair off or seam to pair off cos they are in the tank together and are the same breed. fish tend to stick with there own kind so 2 males will stay together, look like a pair but are not. Its just a friendship that will also show signs of challenge too. Its these challenges that a lot of people mistake as pairing and mating. as far as pairing off to mate, 9 months +.

When to separate due to bullying is a hard one. they can get very rough with each other, chasing can be fun/challenges/sexual choosing whats going on is down to you watching. you also need to take into account that a fish swimming into a rock/log/tank cover and becoming damaged isn't intentional. i would move a fish if it sustains constant intended damage. side attacks to the gills leaving small holes is common and can look like HITH. constant fin nipping or if the chased fish is becoming stressed over time. i would also give them a few days and accept the damage over these days if a new fish is introduced to a existing tank. Bullying won't stop after a few days and will go on. Watching and trying to understanding whats going on is the answer.

my two oscars grew up together and get along well before. at the early
stage of chasing, lip locking, i don't mind... few scrape and lost scale
will be fine... however, when bullying shows the missing cheek muscle,
i have to move one of them to give healing time with the injured.

a few days later, i switched partner and no issue yet until now...going
two weeks already....

My two oscars, a tiger and a red have been 'fighting' for some time now. ive had them for three months or so, and the aggresion started just over a month ago. the two are now around 3 inches and growing. the tiger seems to be the dominant one, charging and chasing the red, along with the two trying to liplock. lately ive noticed the red trying to stand up for himself.
i tried the net method but gave up because soon as id get up to try to stop the fighting the both of them would swim to the glass thinking its time to eat
when i notice scrapes or missing scales and such i melt a couple table spoons of freshwater aquarium salt in hot water and add to the tank. the wounds are quickly healed after a day or two.
today i actually took out this big roman colliseum thing that was taking up alot of space, probably being the cause of some of the wounds during chasing and what not

I'm facing the same issue with my juvies, I've had to move out the 2 tigers as the dominant red was giving them a hard time. he's cool with the other red though, seems like he prefers his own kind for now.