correct me if im wrong, you run your od into an amp which you already set to a slight overdrive, in that case.
you'd need a footswitch for your amp (either to off the effect, or switch to a clean channel) and just switch effects by stomping the od off and turning the chorus on. using an a/b box for such a thing is a slightly over priced option for a quick tapdance on your board, unless youre planning on using two long effects chains. or some other chim-mer setting.