If a story has a well developed plot, I can ignore most grammar mistakes as long as the story can gain my attention before the mistakes piss me off. If it doesn't it's the back button. I can't complain about punctuation as I am still learning to use it properly. I will almost always avoid something with numerous spelling errors. If the writer can't take the time to run spell check, I can't take the time to read it. One or two is no big deal. I can be a little OCD about misspelled words and even I make mistakes and miss something occasionally.
OOC will not necessarily kill a story unless it is so very far from the original character, that it totally kills the image I have of that person.