Either use double sided satin ribbon or you can line the back with parchment using double sided tape to make a layer or parchment (or wax paper) between the ribbon and the cake-if that makes sense.
If you let the cake set up all night and crust well, then you can put the ribbon on the next day without soaking up the oil. I have done this alot.
I run a strip of wax paper that is just a teensy bit smaller than the ribbon around the cake and then attach the ribbon to the wax paper, and itself, with a dot of buttercream or royal icing to secure. Works very well.
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