Freezing a cold blooded animal is not humane,,, sorry so many think so
Just because it stops moving doesn't mean it's dead. Sever the spine right behind the skull then open a fish up so you can see the heart beating and start an egg timer, I think many would be amazed and a little horrified at how long it remains active. Using ice water until the core temp reaches the 30's brain function remains. I'm not a card carrying member of PETA but aquatic animals can experience distress, panic and (imo) pain. Having an Oscar your good to go OP. For others that don't have a predator tank consider the same chemical used to tranquilize fish for surgery. At a higher dose it puts them down with only mere seconds of distress and the cost is minimal as it relates to what most spend on the hobby.
a beating heart doesn't mean it's still alive though. a severed spine is instant death. electrical impulses keep the heart beating for a while. Regardless, the fish isn't experiencing anything because the brain has been removed. We can of course postulate on how long the brain remains active and the fish aware; like how we hypothesize the human head, after being severed with a guillotine, can last up to 8 seconds.
I love the back and forth on humane ways to kill fish/fry. Rest assured, if you feed living fry to an oscar, they're going to be alive in its belly for a bit, especially if it's a larger one that is just going to suck them right down.
I'm not saying don't do it, I'm just saying it's odd to think it's more humane than chucking it into freezing cold water and shocking it's system into shutdown within seconds. And I doubt fry have much more brain function than, "find food, eat food, don't be food" for quite some time.
Well, yea, you can hit them with a hammer too. But I vote for the oscar.
Some people do that, effective and fast. Here are other culling, um, techniques(?) that I've picked up from discus breeders:
1) Garbage Disposal
2) Put in plastic bag and slam on the cement dump in garden
3) Slam on concrete then flick in the garden with a stick
4) Sever head with sharp knife then chuck in garden
5) Feed to ferret
6) Feed to dog
7) Feed to cat
8) Feed to fish eating snake
9) Feed to fish eating lizard
10) Feed to larger fish
11) Feed to turtle
It's a rather interesting exercise to try and find ways to close the circle in aquatics.
My plant trimmings go to the compost, which provides soil enrichment to the houseplants I take trimmings from to add to my riparium setup--or are aged and used to supplement the usual debris in my tanks to feed snails and my red claw crab.
Excess snails are smashed as treats for my various stock, or dumped in a bag to dry--at which point I crush them to a powder and add 'em to my homemade shrimp foods to provide high mineral/protien content.
Excess endlers--including the gradual reduction of my female population--get chopped up and served up to various fish, shrimp, the crab, my cats and--if any is left--dumped in a jug on the balcony that holds an ever evolving fermented brew that I use to fertilize my houseplants.