Which has higher priority (Cahn-Ingold-Prelog)?
As mentioned by Yomen , the double bond counts for two atoms with a single bond between them . They are called ghost atoms . This makes aldehyde group at higher priority than the ether .

Wikedia has a very nice explanation on it :


  If an atom A is double-bonded to an atom B, A is treated as being singly bonded to two atoms: B and a ghost atom that has the same atomic number as B but is not attached to anything except A. In turn, when B is replaced with a list of attached atoms, A itself is excluded in accordance with the general principle of not doubling back along a bond that has just been followed, but a ghost atom for A is included so that the double bond is properly represented from both ends.


A triple bond is handled in a similar manner . 

Wikipedia link : Cahn–Ingold–Prelog priority rules