O RLY?, short for "Oh, really?" is a popular Internet phenomenon, typically presented as an image macro.
The original O RLY owl image macro is based on a photo of a Snowy Owl (Bubo scandiacus) taken by nature photographer John White, which was posted by him to the newsgroup alt.binaries.pictures.animals. SomethingAwful photoshopped in the "O RLY?" phrase and the resulting image spread quickly to the /b/ board at 4chan, an English language imageboard, based on the famous Japanese imageboard Futaba Channel.
The owl image macro quickly spread to other websites such as Fark and YTMND, spawning many more image macros involving owls.
More Info: http://en.wikipedia.org/wiki/ORLY
Interestingly, "Orly" is also the name of a cosmetics company that make a range of nail care and beauty products, especially Nail polish.
Even more interestingly, Orly is also a commune in the southern suburbs of Paris, France. Don't believe me? Here is the website of the Orly City Council: http://www.ville-orly.fr