What is the origin of the name oxer for a show jump fence?

I asked the same question to a world class jumper and he did not know.

On my own research investigation I came across "ox fence" which is a strong fence

consisting of a ditch,hedge and rail on one side. I would surmise that the double rail fence (show jumping -oxer) is called an oxer because of its similarity to a strong fence to keep in oxes .

I am continuing my investigation and will report any new findings.