I switched from Zaino Z-5 to OOS and OCW combo. With my particular car (2010 Prius Winter Gray which is a medium metallic gray) the OCW makes the car look too light gray. I prefer the look of the Z5 as it makes the car look darker and the metallic stand out more (just my preference). My question is, now that I've layered my Z5 back on can I follow up and layer OOS on top of the Z5?

I personally don't see any issues that you should have with this type of application. OS is designed to be applied as your last product and I am sure that the Zaino is well cured by now so there should be no issues.

Anthony

From a hobby detailers viewpoint, wouldn't applying the Opti-Seal over the Z5 preclude applying more Z5?
If you like the look of the Z5, why not just use it with out topping it with anything.
My usual procedure which isn't necessarily correct, was to use only a carnauba on vehicles where I preferred the look of the carnauba and only a sealant where I was happy with the look of the sealant. If I didn't care for the look of a LSP, I changed LSPs.