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 wrong with the skin you're using, please post a comment with a link to the skin, and a screenshot showing the skin on both your action bar and OPie. My goal is that OPie buttons should look as much like normal action buttons as possible.
* There is now one option, for controlling the visiblity of OPie's color highlighting, because some Masque skins have really ugly "highlight" textures. If you don't like how the color highlight looks, open Addon.lua and Notepad and change the value of the SPECIAL_COLOR_ALPHA variable at the top. A value of 0 will completely hide the coloring, 1 will make it fully visible, and anything in between will make it partly transparent.
Updated for WoW 6.0
Updated for changes in OPie version series Orange and Plum.
Updated for changes in OPie version Nougat 1. Not backwards compatible with older Masque versions.
Added an option (Lua config only) for controlling the opacity of OPie's slice color highlighting, due to some Masque skins having ugly highlight textures. 1 = fully opaque (shown), 0 = fully transparent (hidden).
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