So I'm running OSX and I've created a Debian system into a Virtual Box machine. I want my computer (OSX) to access the Internet through it instead of using Airport. Is there a way to do that?
I'm assuming that you want to use the Debian machine as a web proxy. If so, then you'll first need to make sure that you set up bridged networking, so that the Debian machine is on the LAN. When you do, it'll get an IP address from the LAN (if DHCP is running), or you can give it an IP address.
Then, you need to set up suitable proxy software, like a web proxy, on the Debian machine.
Then, configure the proxy on the Debian machine as the Internet proxy on your OS X machine.