Well, that covers all the oxen songs I know. I hope to learn more this summer at Tillers, a school which teaches animal traction, timber framing, and blacksmithing, amongst other things. You can find them on the web.
The Midwest Ox Drovers Association (also on the web) holds its annual gathering at Tillers this year, June 24 & 25. Classes, clinics, and pulling competition are scheduled.
Let me second the recommendation of Rural Heritage. They maintain a great website and publish a highly informative periodical.