First challenge, if you fill it up to a depth of 42, buying enough salt for approximately 6000 gallons of water. At even roughly $40 per 160 gallon bucket, assuming you can get a "good buy" if you buy bulk, you'd have to buy around 38 buckets of salt, which would cost $1500 just for salt.
Circulation, that would be fun. While a pool pump can do some, it wouldn't be near enough for a marine tank, even just a fish only tank. And the salt would play havoc on the pump and other metal components of the pool framing. It probably wasn't designed to for saltwater use, so it's will probably corrode pretty quickly. And the pump may even have metal parts exposed (brass, copper, stainless steel, etc.) that could be either toxic or corrode quickly.
And heating it in the winter? Forget it, would cost an arm and a leg to keep an above ground pool warm in the winter. Below ground pools are extremely expensive to heat and they are insulated on the sides by the earth. You'd really need to build a greenhouse over it to help on this one.
What about cooling? Without some shade cover, the water will get really hot out baking in the Oklahoma sun during the summer. Will probably get to hot even in the shade. Likely way too hot for anything to live in it for long.
In the end, I think it's completely impractical to use a pool for a saltwater tank in Oklahoma (and many other places as well). You might be able to get by if you're willing to build a greenhouse over it, but a greenhouse that large will be rather expensive. And you'd have to check to make sure that everything would be saltwater safe, pumps, filters, heater, etc. I'd estimate that unless you have many, many thousands of dollars to spend (and the free pool would be nothing compared to the total cost) you'd be disappointed in anything that you did end up with.
When I was in Hawaii they had some really nice salt water ocean pools, that were concrete and aquascaped that looked great. They had the advantage of pulling in thousands of gallons an hour of natural sea water to manage it, so no heating, cooling, or filtration. And the fish they had to put in it began at about 8", anything smaller and you couldn't see them. It had some of the largest tangs I've ever seen outside the ocean (though it's practically the ocean, just a hundred yards from the beach). Had other fish up to a couple feet long in it as well, but it was a multi-million dollar resort that had the resources of the ocean right there. Of course it was hard to see the fish in some cases as you could only see them from the top, unless they were swimming near the surface on their side.