This is a pretty interesting spot.

On the one hand, OMC is leading into you on the river... usually indicates a strong hand.

On the other hand, OMC's bet is tiny. In my experience, when OMC has a hand, he bets huge. This looks like a blocking bet.

I think its extremely unlikely that villain is limp/calling from UTG+1 with 64s, 54s, 42s. So I think we can eliminate these hands from his range.

Hero has the Q and T, so Villain cannot have JTs, T9s of hearts.

I think its pretty unlikely that Villain has the AX or KX flush since his river bet is so small.

I'm leaning strongly towards J9 or the many non-heart combinations of T9, 98, 87 and 76.

I think this is a snap shove on the river and see if Villain decides to take a stand with a bluff catcher. If Hero takes too long to make his river shove, OMC might reflect on his bet and level himself into a fold, so timing is possibly important.

Thanks for the input, all. This has been a good discussion.

Results:

Like Lapidator, I immediately saw the $80 bet sizing as being an indication of small value/blocking bet vs absolute nuts. I quickly announced a raise and pushed forward two red stacks for $200, hoping the old man would call with a range as wide as oddly played sets, Tx, and lower flushes. The old man just called and turned over A 5. Not sure if he couldn't see the chips I had behind, forgot that he had the nuts, or forgot that he had the option to 3bet. In any case I got to keep roughly $55 behind.

lol at calling. Min raise/fold still seems pretty good.

Guess his range was so tight that he wasn't even shoving with the stone cold nuts.

Is it possible for Alzheimer's to cause one to forget a thought process that took place 20 seconds ago?

I picture it like this:
"Ooo, got the nuts. Need to bet."
"Got raised? How did that happen?"
"Pretty sure I had a good hand. Ok, I call. "

After the session I was pretty annoyed that I value owned myself in this hand but failed to get value OTR in the other hand with set vs JJ overpair on a board of 9h 6h 4h Tc 7c.

I think I might be being too critical/somewhat results oriented. Do I seem to be too nitty by checking it back on that other hand? The more I think about it the more I think JJ is one of the rare hands that would call a bet while losing...he can call pf with TT/99/88/77/87/8T and c/c 3 streets with those hands if they include one heart. He isn't paying me off with AhJx type hands (although maybe c/r bluff occasionally). Probably always leads if he does actually have a flush. Not sure if he calls three streets with something like Jh9c, think I might only get called by overpairs/2p+. Any quick thoughts on that hand?

The problem is your calling range is only flushes, and you're not taking into account that his line is more consistent with a straight than a flush (especially given hero's strangely small bets, which make villain a bit more likely to donk his straights on the river). Also due to his river bet sizing I think more weight needs to be put on the smaller flushes than the bigger ones, since most players want to get paid with these hands so they bet big. In order to get called by a better hand, the villain first has to have one, and given his betting I would say it's fairly unlikely. I would have pushed, and I think this just turned out to be an unlucky cooler.

Just because I couldn't see how V would get to the river with an 8, or how he could play a flush so passively, I would have b/f otr with the 666. Depends on V though, most guys are incapable of representing anything, let alone c/r otr to represent a made hand. Those guys are also probably not calling with JJ though, I know it's thin.

Maybe bet something tiny like 25. I do that a lot when someone's ai in the main pot, but I'm in a sidepot and hit the (near) nuts. My V in the sidepot will get to see my hand regardless when I need to show it for the main, so I've bet really small otr a few times. When you do that every V convinces himself you've bet that small because you have the stones and are begging to get paid off, so I wouldn't worry about it looking like a blocking bet and getting raised off of it.

Villain in that other hand had a VPIP/PFR of 57/5. Had previously seen him check with the nut flush OOP OTR (hit OTR) looking for a c/r. Other than that, only time I've seen him push action was a hand in which he called SB's raise in EP, 3bet OTF over SB's flop raise after flopping bottom set on a two tone flop, and shoving turn after SB called his flop 3bet and three flush came OTT.

He touches his chips for 10 seconds and then checks on the river in the hand I played, and I think this was a big tell that he had some showdown value that I might beat with a set.

One of the two biggest tells lol. Getting chips ready to bet then not betting (in turn or out of turn) means they're 1) not calling 2) calling but hating it.

And I love it when a guy is faced with a big bet and says something like "Damn it, that flush hit you didn't it, I got so unlucky..............I raise, I'm ai."

Saw this again recently...

Final board was J9249 with the 9 on the river bringing the flush. First to act checks, then someone bets and another one calls. First to act says, "huh... you guys hit the flush... oh well, let's all show our flushes... I'm All In". He get's snap called in 2 spots by 2 (crappy) flushes. Of course he had J9. LOL!

This is very true of old nits of llsnl.