Will these cats ever tolerate each other?

About 4 months ago my husband and I adopted a 6 year old neutered male cat. We already have a 5-6 year old spayed female cat, which we've had for about 3 years.

For the first month after we got the new cat we kept him in a separate room. To expose the cats to each other we would feed them on different sides of the door and open the door a crack to let them see each other a few times a day. The new cat (NC from here on) didn't have a problem with her. He grew up in a house with two dogs, so I guess he's accustomed to having other pets around. Our old cat (OC from here on) is a different story. She would growl and slap at him every time she saw him or was near him.

As time went on we'd let NC outside the room during times we could supervise him. OC didn't tolerate him any better at all. She would constantly growl and hiss at him and sometimes scratch him. We would separate them into different rooms as soon as they started physically fighting.

Over time, NC stopped being scared of her. Now he almost seems to egg her on and will sometimes chase her under the bed to terrorize her (or so it seems). I have never seen him scratch her, though. She seems to be the one doing most/all of the scratching.

Eventually we let the cats out 24/7 together because it seemed like it was getting better. Then it got worse. A month ago we started keeping NC in a separate room again, and again only letting him out when we could supervise. The fighting is not any better at all. Sometimes it seems like it's getting worse! Every 20 mins or so when NC is let out there is a screaming, violent cat fight going on. And when they're not fighting, OC seems stressed out and angry by his mere presence.

Will these cats ever learn to tolerate each other? Is there anything we can do? NC is understandably getting frustrated being kept in a small room most of the time (he has a lot of energy), but I don't know of any other solution. I don't want to have to keep him in this room all the time.

Sorry this is long, but I wanted to give as much detail as possible! Any help is appreciated!

Thanks so much for the reply. I think they actually do hurt each other when they fight. They have both had scratches on their noses and we saw dried blood under the bed where they usually fight.

We live in a 2 bedroom apartment. I'm beginning to think you're right about letting them duke it out and figure out the pecking order. I just hate hearing them fight like that.

We have had days where we keep OC separated and let NC have the rest of the apartment, but he's not as trustworthy as OC. He's pretty high strung and likes to jump on things, eat dried flowers, climb the doorframe (literally), etc. so we don't feel as comfortable letting him out unsupervised, although we will have to eventually do that. He can't live most of his life inside a small bedroom.