[Mischback]

Originally Posted by Xruptor But wouldn't that make two styles show up rather then one?

What do you mean? Yeah, you would have two styles, but why care? Take a look at my main layout-file and scroll directly to the bottom.
You will notice, that I define one style for every single unit.

You could go with something like this:

local function yourStyle(self)
    -- do beautiful things here
end

local function yourPlayerCastbar(self)
    -- your player castbar code here
end

-- snip

oUF:RegisterStyle('oUF_yourStyle', yourStyle)
oUF:SetActiveStyle('oUF_yourStyle')
oUF:Spawn('player', 'playerframe'):SetPoint('CENTER', UIParent, 'CENTER', 0, 0)
-- spawn more frames,if you like
oUF:RegisterStyle('oUF_yourCastbar', yourPlayerCastbar)
oUF:SetActiveStyle('oUF_yourCastbar')
oUF:Spawn('player', 'player_castbar'):SetPoint('CENTER', UIParent, 'CENTER', 0, 40)