Bond a piece of sheet steel or aluminum to fill where the hole in the side is, and then patch the liner. That should work fine if the side of the pool is powdercoated, baked paint, Kynar, etc., so you have good bond strength between the paint finish and the steel, because you'll be bonding to the paint finish. Unless it's something hard to bond to. I really like adhesives, so that's my preferred method. 2-Part structural adhesives for auto body repair will work great, and are fairly cheap.
Or patch it with sheet steel or aluminum and pop-rivet it from the inside (smoother rivet head), if you can access the area between the liner an the the structural side of the pool. Should be no problem if you have an 18 inch hole in the liner, because the steel patch can go on the outside or inside, so you just need to pull the liner back at the hole enough to pop rivet all around. Cheaper than adhesives, requires drilling a lot of holes.
Either way, you need something to reinforce the bigass hole, and then patch up the liner.
My mom tore down their above ground pool a few years ago, and it was there when they bought the house in '92. I've patched that pool, replaced the liner, done all those bullshit things I never wanted to do again.
Are we talking about a traditional above ground pool, with steel or aluminum sides and a vinyl liner, and an 18 inch diameter hole in the steel side?