Are ozzy possums full of disease? I had an ozzy possum sneak up on me outside a pub and it seemed friendly. I was told not to stroke it cos if it bites I'll catch rabies. Still not sure if they were being serious.

There's no rabies in Australia, so no, you can't catch it from possums or any other animal here.

It was an American who warned me. Sounds like their possums are cunts. Can't blame 'em.

Ah, yeah I can't speak for American possums, not having ever encountered one as I have not yet visited the USA.

Do they do anything useful? Eat lots of grass? Keep rats away? Assassinate cats?

They make me feel happy with their cuteness. :)

And it kinda looks like you can keep it at home like a cat,watching some netflix with pika on the lap.

American opossums (Virginia Opossums) are scavengers / opportunists, they clean up road kill and the food out of my cat's bowl.  They can be trained. Some of them are pretty dirty.  They are not aggressive in the least. Worst I've seen them do is hiss at you if you have them cornered and scared.  I've tried to shoo them off the porch before with a broom and even though I'm shoving them with it / lightly tapping them they don't bite at the broom.

They don't seem particularly intelligent.

Opossums in America almost never carry rabies.  It is possible, but very very rare.  It is due to their low body temperature.  It is believe that it makes them a poor host for the virus.  However, they can be mean little shits.  I wouldn't recommend going near a wild one.