So far the only time I’ve seen this stocky heron was on my trip to Hawaii over ten years ago.

I had decided that after passing my qualifying exam for the PhD program, I was going to treat myself to a trip; I flipped a coin and it was Hawaii (the other choice was Italy, but since I don’t speak Italian & wasn’t up for a 12+hour plane ride–I decided to stay within the states).
I then picked the big island (Hawaii) as the island I would visit, as it is home to the Volcano National Park, a winery, and numerous other things to do.
I stayed at a hotel that was basically on Hilo Bay with an attached restaurant (made dinner easier).
I think it was on the second afternoon/evening that I noticed this stocky heron peaking through the fronds of a coconut tree.

These are birds that you will usually only see at dawn/dusk and in the evenings–hence thier name of night heron.
The adults are a light gray in color with a defined black back and crown.
The young are brown with white spots on the wings and blurry streaks on the chest before they go through a molt to the adult plumage.
In terms of their beaks–the adult beak is all black, while the young beak is yellow and black.
In terms of their habitat–these birds are wetland birds (as are basically all members of the heron, egret, and bittern family). Therefore they can be found in typical North American wetland habitats including: estuaries, marshes, streams, lakes, tidal mudflats, and reservoirs.
They can be found throughout most of North America (with a few exceptions such as Alaska, the extreme northern parts of Canada and then some parts of various states).

Most of the Midwest is within their summer (breeding) range, and that includes Oklahoma.
I would think that in order to possibly spot these birds I would have to possibly go to one of the larger lakes in the area (though they may be at Boomer Lake), closer to the evening and carefully walk along the edges that are lined with trees to see if I could spot them roosting.
In terms of their diet–they are opportunistic feeders (possibly because they are feeding t night which helps them avoid competition with other heron and egret species that use the area during the day).
Their diet consists of terrestrial, freshwater, and marine animals (all depending on their location). They have also been known to eat small birds, rodents, turtles, and snakes in addition to the usual diet of earthworms, insects, crayfish, crabs, and clams.

The only time you will possibly spot them feeding during the day is during the summer (breeding season), when they need the extra energy for nesting and feeding the chicks.
While they currently have a semi-stable population, these herons also had to make a comeback after the banning of DTT and are still susceptible to pollutants such as pesticides, PCBs, and heavy metals.
Scientists are now using black-crowned night herons as a marker for environmental quality. This is because they forage at the top of the food chain, nest in colonies, and have a wide distribution, plus they tolerate disturbances such as traffic near their nests (where s other members of the Pelecaniformes order do not).
Photography goal will be to get a picture of a black-crowned night heron within the continental US, plus a picture of one catching its evening meal.
Reference: https://www.allaboutbirds.org/guide/Black-crowned_Night-Heron