Yes, they certainly can. They can be annoying about every game, not just Oblivion. When I got here, people hated Morrowind. It was actually kind of traumatic to me to be exposed to the hate directed at the game. I was pretty new to game forums and was not used to the way gamers express themselves over the internet. But every game that has ever been made has had its detractors. You cannot please everybody.
The way I see it, people who buy a game are entitled to express their opinions about the game they have purchased. Some of them have very intelligent, thoughtful critiques to make and I have found that it is worthwhile to consider those opinions carefully. Intelligent critiques have changed my own opinions more than once. Some of the criticisms directed at Morrowind back then, I now realize, were justified.
I enjoy reading thoughtful opinions that differ from my own. It is one of the ways in which I grow and develop as a human being. But when I encounter a critique with little or no thought behind it (i.e. "THIS GAME svckS!"), I just ignore it. It's not worth my time or what little is left of my mental health to get upset by ignorant opinions. Life is too short. I have better things to do than to waste my time thinking about how stupid other people can get, no matter how true that might be.