Hello, and welcome.
While I have not used pppoe, a moment of Googling found
http://www.openbsdsupport.org/obsd_dsl.html which was written for OpenBSD 3.5 and updated for OpenBSD 3.7. Proceed with caution, as there were six releases and three years between it's publication and the release you are using now. I have not read the document, but it shows altq being used with tun0.