Wikipedia suggests, with a supporting cite, that full-grown adult bulls are too large and aggressive for orcas, and will even attack them or at least threaten them.
You realize that an Orca can weigh 8,000 lbs., right?
22,000 pounds, actually.
King cobras can kill elephants, which weigh up to 12,000 pounds.
It’s not impossible that a very large venomous snake might have enough venom to kill a small orca, or at least make it weak enough to drown.
Elapids deliver much more venom per bite than other venomous snakes. King cobras, for example, can deliver 500mg in one bite. By contrast, sea snakes seem to deliver much less venom, though their venom is far more potent. If I’m doing the math right, you would need 396 mg of even the most potent snake venom to kill a 9000 kilo orca., but sea snakes (this one at least) only deliver tiny quantities per bite (4.0 mg.)
Where are you obtaining mg venom per bite data?
Sea Snakes are an elapid Krait species so much smaller than the KC that it should not be surprising that KC venom delivery per bite is much greater.
However, numerous vipers (e.g Diamond Backs and Gaboon Vipers) are more massive even than the KC, and I believe I have read that the GV has the highest venom per bite of all.
Getting back to the terrestrial elapids, you certainly wouldn’t want to get bitten by a KC, but the Black Mamba (an African species), the Taipan (Australian), and terrestrial Krait (South Asia) deliver bites which are virtually 100% fatal if untreated, and I think the untreated survival rate for a KC bite is small, but non-zero.
According the the University of Florida Dept. of Wildlife Ecology and Conservation"
(from link)
Snakes have some control over venom delivered, and I expect a DB could inject all 850mg if it wanted to.
LD50 for the eastern diamondback is more than 1 mg/kg, so that’s still not going to be enough to kill reliably (though we seem to be closest to bumping off our orca there.)
But at least we can rule out the Giant Octopus. 
In the end, the octopus is killed by Gleason’s pair of killer whales.
Hey, I have seen that movie! Nifty!
I thought I’d heard years ago about pods of dolphins attacking (or maybe defending from?) killer whales. Maybe it was an orca who tried to take down a dolphin but was then beat to a pulp by the dolphin’s friends. I don’t think a dolphin would intentionally attack something as big as an orca, even in a group.
The most deadly species of all: humans. They just have to drop a nuke on them. https://c4.staticflickr.com/4/3015/3128676813_120f8ee156_b.jpg
I guess the edge length of a fin would fit, too, but I’m further guessing that the skin is quite tough there.
Are the tentacles & suckers of a colossal squid strong enough to latch and hang onto a struggling adult orca, if the squid wanted to pull it down and drown it?
But the only way to be sure is to do that from orbit.