OPie dropped built-in Masque support in version Lime 1. This plugin restores it. Obviously, you must have both Masque and OPie installed. There are no options. Just install it, and you will once again be able to use Masque to skin OPie.
Also, you will be able to modify the numeric cooldown text using OmniCC (and if you don't have OmniCC or something like it installed, you won't see numeric cooldown text at all).
Only tested with my personal (unreleased) Masque skin. If something looks off with the skin you're using, please post a comment with a link to the skin, and a screenshot of how the skin looks on OPie vs how it looks on your action bar. My goal is that OPie buttons should look as much like normal action buttons as possible.
Updated for WoW 5.4
Updated for changes in OPie Lime 6
Updated for WoW 5.3
Now prevents OPie from blacking out slices while on cooldown
Fixed an error with using Broker plugins as OPie slices