Is answering Okay to the question, "How are you" rude?

No I don't think it's rude.
"I'm fine, thank you." would be good manners.

I think that's a good answer but all depends on how well you know the person who asked and how much you both elaborate, or how feel at the time. I tend not to say express by feelings unless it is warranted.

No its on the casual side but it certainly doesn't run to the rude.
In some formal settings, it may come off as a bit unpolished but that still isn't rude... unless the person knew better and did it with malice, then that would be rather rude.

"I'm well, thank you" or, "Fine thank you", followed by, "how are you?" is probably the most polite. OK is, well, "ok" - not bad, not great. It can come off as not really being o but more like, "well, not great".

I think what is rude is to completely unload on someone who asks this in passing as a greeting, or to be impatient when people actually do tell you more than you wanted to hear in response.

I do think that in close relationships, when the other person is really trying to engage you in a discussion because they feel something is not quite right, the answer of "ok" is dismissive, and in that way is rude. If this is your situation (your Mom finds it rude, your boyfriend, whatever), I would think that answering, "well, you're right, some things are bothering me" might be better and if you don't feel like talking at that time, just say, "I'm really tired right now but we can talk about it after dinner (or tomorrow night, or whatever)". The other person needs to respect your boundaries, and if they don't, you will be justified in going back to ,"ok"!