Saša Zorc

I don't think he should.

Your river betting range consists of bluffs, boats and JTxx. Since he has the nut low boat, for the raise to be profitable he would need to get calls from straight more often then he runs into a better boat, and I don't think that is happening barring some very specific reads.

Quite interesting hand, here are my thoughts on it...

There are a few problems apparent:
a) With this bet size, you need him to fold more then 65% of time to show immediate profit
b) His range should be mostly value bets. Based on the fact that there is little unshowdownable stuff in his range once he check calls + he probably checks weaker but showdownable hands on the river. I am not sure how loose he value bets here, but think it would be safe to assume the majority of his value betting range are flushes.
b) You are repping very thin: nutty hands that you played in an atypical manner to exploit his river leading frequencies. Which is also problematic as those frequencies can also be exploited by bluffing more in spots like this, and he is probably aware of that.

So basically Isildur has to fold something decent at a high frequency for this to be profitable. I am not convinced he would, Isildur is not known for his big folds and you are repping very thin here. But in the end it all comes down to stuff like game flow, how he perceives you and how he expects you to adjust. If those variables are favorable then this can suddenly become very profitable.

Not really Abu. Folding here is exploitable because;
a) we are near the top of our range
b) we are blocking a lot of value betting hands for villain

Those conditions don't apply if we have, say, JTssxx. Folding stuff like sets and straights here at a high frequency can still be unexploitable. Therefore there is room for bluffing in villain's shoes.

Since you asked about GTO play:
If you are folding the second nuts here, then you are folding almost all of your range in this spot which is extremely exploitable. So yeah... calling here always is the GTO play for sure.

I play predominantly heads up PLO and my HU hud includes:

SB VPIP/ BB VPIP/ 3bet/ Fold vs 3bet/ 4bet
Flop Cbet/fold vs flop Cbet/Turn Cbet/Fold vs turn Cbet/River Cbet/Fold vs River Cbet
Flop check raise/Fold vs flop check raise/Donkbet/Fold vs Donkbet/Leead turn lead after donking
Bet vs missed Cbet in position/Bet vs Missed cbet oop/Bet vs missed turn Cbet in position

Your hand is obv stronger vs his range on the river then it was on the turn, but one piece of key information is missing:
How often does he give up on the river with his bluffs? If rarely then river is a clear call.

What Phil Long suggested is the standard line for me.

I agree, given the villain description, the majority of his range should be KK.

The way you calculate how often the bluff needs to work to be profitable: The amount you but divided by the size of the pot after your bet before any other action.

To use your example: betting 13100 into 9972 you need a fold in 13100/(13100+9972)=0,568 to be profitable. But this super basic poker theory and not omaha specific at all.

Villain's range for calling flop is mostly composed of different straight draws, two pairs and sets. On the turn all draws now either have a straight or 2pair+. If you bet turn straights and sets aren't going anywhere, and neither is anything that turned a flush draw. Two/three pair might go away. It is hard to estimate how frequent is each of those hand types without doing some propokertools simulations, but if I had to guess, I would say the minority of his range is folding turn and this is not the best spot for barreling.

Also worthwhile to note is: when you decide to barrel this, I would rather do this with some hand that has better equity against their calling ranges then AA, like having the turned flush draw.

Afterthought: a lot of the turn calling range for villain still only has a bluffcatcher on brick rivers possibly making this a good spot for 3barreling if you decide to barrel(?). Not sure about this one.., I rarely do it, but noticed some other regs are successful with it.

... I would only reraise/fold this if I was convinced that villain will rarely 4bet bluff here.

I am passionately against 3b/folding as standard. The thing is, if the villain is the type of player that will combat 3bets mainly by 4betting (and most tourney regs are) you are essentially turning your hand into a bluff. Since AJ can no doubt show a profit calling here, turning it into a bluff destroys any value the hand has and does bad things to our 3betting/flatting ranges as well.

Frequency wise, I think i donk like 10-15% vs unknowns, adjusting a lot mainly based on opponent cbet%.

Some of the hand types that are good for donk betting, but weren't mentioned yet:
-Hands that are just a bit too weak to check call (Includes draws and weak top pair type hands), works best when you aren't drawing to the nuts otherwise check raise might be better
-Hands that can get it in on the flop but are hard to play turns with if you check raise (ie set on a very slippery board), works best against people who fight your donks by raising

This post tilts me so hard...

From the information available to you, you cannot possibly conclude that his range consists of just AA.

Let us examine what we know:
1) Opponent folded 3 orbits in a row
2) Opponent open shoved 20bbs on bubble

Given that info, It is still much more likely that his range is QQ+,AK or even wider. Think the other posters here argumented that very well. Also significant is that you are ignoring some important information available to you, such as:

3) This is a 5k donkament, your opponent is not likely to be a nitfish
4) You could easily find out what kind of a player your opp is by checking PTR/OPR