This morning, our Oscar wasn't as active as usual. Upon turning on the tank lights, I noticed that his left eye was bulging our further than usual, and had a partial 'membrane' over it. He was also swimming erratically, possibly because he can't see well. No change in diet, water parameters all well within bounds, no changes to tank interior of any kind. I fed him, and he eventually saw the food and ate it. He is staying in his 'cave' (flower pot) and not moving around much at all.
I can only surmise that he has injured the eye in one of his 'bumping' actions, as Oscars do. Is this an injury that will heal itself, or is it a symptom of a condition? He seems bewildered and disoriented. Is there anything I should do?

It sounds like Popeye disease but this is normally related to poor water conditions and I think you may have noticed something before now so it could be some kind of injury. I would just keep an eye on the the situation [no pun intended]. If it doesn't get any better within a few days, all starts getting worse, you may have to medicate the tank.I assume that you have checked for ammonia, nitrite, nitrates and pH?

Hello, I had no personal expierience with this problem, but searched for some info.
This may be your problem:
www.fishlore.com/aquariummagazine/dec07/fish-popeye.htm

So, increase your waterchanges to begin with.
How much and how often do you do your WC?

Do you have any Plecos in your tank. I had a pleco blind an O in one eye before.

This is pop-eye. The most common causes are injury or bad water quality.

Do you have any other fish in the tank and are they aggressive

Have you tested your water, can you post the results for ammonia, nitrite and nitrate please

In the meantime do daily 25% water changes and add an anti-bacterial (im in the uk and I use myxazin)

Dont worry if your oscar doesnt eat, they can go 2 weeks without eating easy

based on your description, your o indeed is suffering from pop-eye.
disoriented and it is as good as blind. but pop-eye is common to gold
fish and beta, i am not familiar with oscar having such problem.

having said so, you need immediate water change but that would not guarantee
that your o would not lost its eyes.