3 days a go i bought an archer fish to put in with my 8 inch Oscar, the archer is roughly the same size but oscar just keeps chasing him it has kept going because when i got home today i noticed the archer has cut his mouth were he has bumped into the tank when he was being chased by osacr. i like the archer and dont want to get rid of him what should i do or is that the only option please help

Unfortunately you have two options. You can either leave the fish in there and hope things calm down, or if the situation is completely untenable then you may have to Rehome the fish. I'm afraid we haven't got any advice that will make things work for you, it will either work or it won't

Did you tell the shop you were putting the archer in with an oscar. This is a very very bad and stupid idea

Oscars are fresh water fish and archers are brackish water. Brackish water is inbetween fresh and salt water. In nature you find brackish water where fresh water rivers meet the sea. Either get another tank or rehome one of your fish ASAP (tomorrow would be a good time!!!!!!!!!!)