The code that disables the default cast bar is near line 381 of <oUF/elements/castbar.lua>:
Code:
if(object.unit == "player") then
CastingBarFrame:UnregisterAllEvents()
CastingBarFrame.Show = CastingBarFrame.Hide
CastingBarFrame:Hide()
Just comment out the last 3 lines of that section (the ones starting with "CastBarFrame") with two dashes each, like this:
Code:
if(object.unit == "player") then
--CastingBarFrame:UnregisterAllEvents()
--CastingBarFrame.Show = CastingBarFrame.Hide
--CastingBarFrame:Hide()