To my knowledge only online applications are received.

http://www.qatarairways.com/global/en/careers.html

But how about calling their main number in Doha or try to approach them for an "on site" application. The main location is

Qatar Airways Tower
(Next to Al Manna Building)
Airport Road

Once again bear in mind that even though you go to the main office they might still tell you you need to go and apply "Online".

Regards!!