We had to take Oscar to the vet the other day because he had a problem with his right eye. He had been unable to open his eye for most of the day and was obviously in quite a lot of pain because he was not his usual happy self. The vet said that he has a eye ulcer and has given us some drops which we need to use three times a day. Oscar is now starting to look a bit better, he can open his eye and is bouncing around as usual. We have to take him back for a checkup at the end of the week but hopefully it will have cleared up by then. As to what has caused the ulcer, apparently it is commonly caused by fighting so I'm guessing one of my other cats is responsible.