Just like anything on the Internets, the response “O RLY?” has it’s roots in hardcore geek culture. The photo of a surprised looking owl with the words “O RLY?” added was originally used in the Something Awful forums as a response to anything that may have been found boring, doubtful or unimpressive. The correct response to “O RLY?” is “YA RLY” with a picture of a brown owl looking stern, and then “NO WAI!” with another owl looking amazed.
source: Know Your Meme