Taking care of my husband who yells OW! or COME ON! over and over again whenever I'm bathing him, getting him in bed, etc. Anyone else?

On the slight possibility that there might be some pain involved..(let's take that first)
Is he taking any medication for anxiety or for pain? If so "pre-medicate" him about 30 minutes before his shower or when you have to make any major moves with him. This would calm him enough that IF he is having pain should allow him to relax so that the pain level will be a bit less.

Often outbursts like what you describe are common and is what I might call a "learned" reaction. From the time we are little when we yell out in pain there is generally a positive response from a caregiver. (when we are little that caregiver is a parent, teacher as we get older the caregiver could be a spouse, or hired caregiver)
Look for facial cues to go with the "ouch" or before the "ouch" a grimace, pulling back, tensing the body. If there is none of that then I would bet you are right that he is not in pain.
The "Ouch" and "Come On" might be just the 2 phrases that come to his mind during the process.
This next suggestion might sound counter intuitive but...
Lower the pitch, tone of your voice. Talk more quietly. During a shower try turning the water off when you can. Wet him down then turn the water off. Soap him up, wash hair and do what you need to do then turn the water on to rinse. The sound of the water is very loud in a tiled bathroom. Also the water hitting the upper torso can be frightening so turning it off might be less so.
last but not least.
There is nothing wrong with a bed bath. If that is more comfortable for him and you can get him just as clean opt for a full shower less often and bed bath more often.

The brain tumor & radiation side effects likely caused brain damage and/or some level of dementia that is causing your DH to shout the way he does. The brain is a complicated thing, so it's very hard to find answers to questions like this; I would be surprised to hear that his doctor HAD an answer, frankly! The doctors often try various medications to see if they will help calm him down, b/c it sounds more like agitation than anything else, since you know he's not truly in pain while yelling OW and COME ON. Maybe he wants you to hurry up? Are you able to ask him if he wants you to rush what you're doing? If it were me, I'd keep his baths down to once a week to minimize the agitation it's causing him; disabled elders don't need a lot of baths to begin with, truthfully. That won't help you with the commode and getting in and out of bed, of course. And medicating him for agitation that only occurs during specific times doesn't make a whole lot of sense either. But then again, you say it's so bad in the evenings which is indicative of Sundowning (which my mother has a VERY severe case of) that maybe a low dose of Ativan WOULD help him. The dose Mom takes HAS helped her get less angry/agitated in the evenings, thankfully. (She gets cussing loudly w/o it)

Also, try putting a set of snug fitting headphones or shooting earmuffs over your ears after putting the earplugs in. That's what we do at the shooting range and if it muffles the sound of shooting pistols and shotguns, it ought to nicely muffle DH's shouts!

I'm sorry you're going through such a thing. Sending you a hug and a prayer for peace and some sort of resolution to this whole situation.

My dad shouts out loud in such a tone that you think he’s fallen or is in trouble go running to him and then he says can you take this cup off me.
in his last hospital stay the nurses found it a bit alarming; he never used the button and he’d shout and they’d go flying over and ask what’s happened. They said if he’s shouty and that’s his way then fine but I think there’s more to it - we’re waiting for a memory clinic appt. I struggle with it. My OH is fab and says calmly and caringly why are you shouting and that helps calm it, but he’s only noticed it recently whereas I’ve had years of it! I’m gonna try the earplugs too!

How do you know that he's not in pain? He may very well be. I know that my husband who had a massive stroke, suffered from seizures due to the stroke and later developed vascular dementia and was completely bedridden developed pain in various parts of his body. He could never seem to pinpoint where exactly it was, and when asked where he hurt, would just say "all over." His pain was always worse when he was moved, whether to the bedside commode, or when being bathed, so although you think that he's not really in pain, he may actually very well be.
Perhaps it's time to have him checked out by his doctor. Best wishes.

That sounds really difficult. I wonder why he does it? Thinks it *might* hurt so just over reacts in case? The ear plugs are a good idea.