If a butcherblock counter top comes pre-oiled, what is the prep work required to get it ready for waterlox?
Oiled with what? Most "butcher block oil" is mineral oil, some has paraffin in it. There might be adhesion problems with these. You _may_ be able to remove enough with mineral spirits to get the varnish to stick.
This might be a question for Waterlox's customer service line.