All MOBAs are toxic. Your success is dependant on your teammates but there is no mechanic in place to prevent or punish people or being bad actors.
I was going to say this too. MOBAs are pretty toxic.
Between MOBA and OW they gotta be pretty close, but I've been playing LoL more than OW recently and I'm thinking OW is perhaps more toxic right now.
There a few big differences that attribute to this:
1.) OW without a true leaderboard makes everyone think they're carrying without really realizing how shit they are.
-- On the otherhand, it does prevent some toxicity against the worst performing players... maybe a wash?
2.) OW has No Ban/Pick phase and is FFA picks during game. Meaning its much much easier to troll or get stuck in bad matchups. Your one-tricks get countered much more easily. As opposed to MOBA where you lock in and can get more favorable matches which never change throughout the game. In OW this leads to people saying "Why didn't you switch?!?!?"
3.) Without items and comeback mechanics OW games can feel much more hopeless. In MOBAs you have alternative ways to make a comeback or "scale up" in hopes you can turn it around. In OW it's simply player skill and if you get shit teammates there is really nothing you can do at all. At least in MOBA 1 player could potentially "carry" the game.
4.) LoL in particular has 10 years of building systems to combat toxicity and I feel like its in a really decent place now compared with even just a few years ago. Lane assignments have made the biggest positive impact imo, as now everyone gets their preferred role (mostly).
LoL still has toxicity, granted, but its definitely been mitigated.