I already have an oscar of about 6 inchs and have been offered another oscar in good health thats about 8 inchs, i have plenty of room in my 125gallon tank but im worried that the two will not get on...is it worth the risk...i dont want any harm to come to either oscar

I would suggest just putting the Oscar in see how they get on. If they have been growing properly, those two Oscars are still reasonably young. The 6 inch Oscar is more than likely less than a year old and not sexually mature. I would imagine the 8 inch Oscar is approaching a year, or may be a little more. Maybe you should come to an arrangement with the place you are getting the Oscar from that you will bring it back if it doesn't work out.

If you are going to try this out i would recommend that you remove the Oscar that is already in the tank rearrange the tank and add them both at the same time. If you put the new one into
the tank it will be invading the oscars territory. Removing it and adding them at the same time will stop this and hopefully prevent fireworks between them.

Another way is to put a divider in. Put a fish either side and after a week remove the divider. This way each fish has its own territory and hopefully they will get on after a while. However some fish never get on and that is the gamble when introducing 2 oscars together

I would expect some grief and fighting between them, it is when one gets bullied and starts to get lots of injuries or doesnt get food that you will need to separate