I can't vouch for Feliway as I've never had to use it, but what I've read in posts over the years on CF it seems to be hit and miss.....some swear it does wonders, others say it doesn't work at all.
First of all, have you had Oscar neutered? At 8 mos. ? the hormones are flowing and if he isn't fixed they're gong to make him naturally more aggressive. If he has been fixed, it may be his temperament. imo, "ginger" or orange tabby males tend to be more feisty and aggressive even when fixed.....usually nice to people but not always to other cats. It's too bad that Finn no longer feels safe now in his own home, which is likely the result of fights with Oscar. He's also demonstrating that he either doesn't want to or can't stand up to Oscar and put him in his place. Even if he did, it would likely aggravate Oscar to continue to attack. As Oscar gets older and stronger the fights would likely get more intense. If I were in your position, I would return Oscar to the rescue, because he's still a young cat that can find his forever home. The only alternative is to keep them separated in separate rooms, and that's hard to live with.
First of all, have you had Oscar neutered? At 8 mos. ? the hormones are flowing and if he isn't fixed they're gong to make him naturally more aggressive. If he has been fixed, it may be his temperament. imo, "ginger" or orange tabby males tend to be more feisty and aggressive even when fixed.....usually nice to people but not always to other cats. It's too bad that Finn no longer feels safe now in his own home, which is likely the result of fights with Oscar. He's also demonstrating that he either doesn't want to or can't stand up to Oscar and put him in his place. Even if he did, it would likely aggravate Oscar to continue to attack. As Oscar gets older and stronger the fights would likely get more intense. If I were in your position, I would return Oscar to the rescue, because he's still a young cat that can find his forever home. The only alternative is to keep them separated in separate rooms, and that's hard to live with.