I know it’s sarcasm, usually. I have a feeling it originated from some l337 speak website, maybe Something Awful…but I’m not sure. A google search is interesting, but useless for my query. A lot of people that play online games use it (CS, SC, Diablo, etc.)
I’ve been using it before that site existed. (Or at least, I’m pretty sure it was before that site existed. Anyone know a way of checking dates on websites? But that genre of webspace popped up a year or two ago.) I first heard it in gaming, particularly in the game Natural Selection which is a half-life life mod. In the game, there is a creature called ‘Onos’ who used to only show up near the end of the game and shows that the alien side is winning. When a marine player (the other side) saw this creature he would yell out ‘ONOS!’ to his team members to warn them. Being an internet game, people started purposefully spelling it wrong and calling it “OH NOES!”
The term might have independently developed of this usage, but that is where I first saw it being used.
One way to test when certain phrases have entered to general geek usage is to check the newsgroups archives. Now google’s usenet search clearly sucks currently, and probably always will, but I managed to determine that the first time someone used the expression “oh noes” in their archive was in November 2000, and it happened during a flame war as a sarcastic remark, so the phrase was probably known and used for a while back then. However it seems to emerge more commonly only in the summer 2002, mostly in gaming oriented groups, so it’s a relatively new (or unpopular) phenomenon compared to, say, l33t or k-rad which have their origins well in the 80s. Though it should be mentioned that google’s database probably doesn’t include most of the messages in Usenet, so this is in no way an authoritative way of research.
I found out that the website was registered on Aug 29th, 2003. I had heard the other usage in 2002. I emailed the web-master and asked him where he heard first heard the term.