Two males don't always get along

I have two tuxedo males, Oscar and the unimaginatively-named Tux, and they have a . . . strange relationship.

I've had Oscar for >8 years, and he's a wonderful high-spirited cat, very active and energetic and a bit possessive of me. I brought home two of his cousins, a male and a female, and they all got along. He and the girl, Liza, actively spar with each other, but they never really fight. The other male, Leo is fairly mellow, but is, in certain ways, in charge of the house.

Then, Tux, a very friendly member of my TNR colony, got a bite to his back leg that got badly infected when he was ~9mos old, so I brought him back here and nursed him back to health, which took 6 weeks of antibiotics. I had meant to get him better and find a new home for him, but we know how that goes. Tux also likes to sit/sleep on me. He is the only cat in the household who was neutered after he was fully sexually mature. (The infection was so serious that the vet wanted to wait to do any surgery.)

Oscar and Tux initially got along, but friction gradually developed; we're at a plateau now. They both like to go outside, and that seems to be where the trouble started. Oscar likes to play, but I think that Tux interpreted Oscar's attempts to play as aggression. Tux seems to go between fearful/timid and aggressive, possibly due to the injury he received from some other cat. Oscar is VERY fast and agile and likes to show off; Tux is bigger and built like a wrestler. They have only had one bad fight, over a year ago, but there have been a few skirmishes. Scratches are rare. On the other hand, they will often sit out in the yard near one another with no problem. They will greet each other with sniffs and lick each other on the head, but Tux is always the first to lift a paw up defensively if he feels Oscar is invading his space. Oscar will sometimes engage in posturing behavior where he walks up to Tux slowly, rubs his head on the furniture, and stares at him. Tux regards this as a threat and tries to get away and hide--even though he's bigger. However, Oscar does the same thing to the female calico Liza, and she interprets this as an invitation to play. They do a little play-fighting, but it never escalates. Oscar and Leo never have any friction.

At food bowls, if I put food in one bowl and Oscar starts to eat, Tux will come over and nudge Oscar out of the way. Oscar allows this. However, if Oscar sees Tux sitting in my lap, he gets visibly irritated. He even jumped up and swatted at Tux once when Tux was in my lap.

So I live with a slightly uneasy truce in this complex relationship. I would love to try to get them to be better friends, but I think I may be kidding myself about the possibilities. I have tried Feliway diffusers, and I think they did help. Unfortunately, having one running in the house played havoc with my sinus problems.

Any ideas? I feel like these two need a psychotherapist....

Personalities are what they are.   Not much one can do to change them.

Sometimes it can takes cats a very long time to accept each other.  Cats work on territory and since Tux is the new comer, Oscar is threatened by her presence.  I always found in my home that there were certain spots in the house that were triggers.  The back stairs in my house and up at the top were a hot spot.  I had to watch my two like crazy.  My two are both males.  One was an indoor only and 5years old.  The other was a stray/feral I brought into the home.  He hasn't lived in a house for years.  Both are neutered.  It took them one year to live peacefully with each other.  It was a constant stalking and my poor resident cat was fearful.  I often had to separate them.  The stray/feral had his own room which he stayed in at night and during the day when I could not supervise.  It took alot of time and effort to get them used to each other.  Now 17 months later, they love to play with each other.  They will nap together in the sunshine and I have caught a few glimpses of grooming.  It is an amazing turn of events. 

I found Composure feline treats or Composure liquid max to help.  I used it on both cats to start.  The resident cat received to help calm his fears and the more aggressive cat received it to help with his impulse to fight.  It worked very well.  I started with the treats, but then both boys rejected them.  I then tried the liquid and it was so easy to mix into wet food and it worked fast and lasted awhile.  You can find this product on Amazon.

Update:

I'm trying out a Sentry Calming Collar on one of the cats as an experiment, and it's the first thing that seems to make a real difference. It's only been a few hours, but it's definitely affecting the dynamic.

I didn't try Composure, but I found another Theanine product at my local pet store. It didn't seem to do that much.

Good that the collar is having some effect.  Let us know how it goes.