What could I substitute for coffee in an Opéra cake
Some sort of nutty flavor perhaps? Hazelnut syrup? I think it should go well with both chocolate and orange. Otherwise I think the cake will be quite delicious even without a substitute, just using plain buttercream. 

EDIT: I just noticed that the cake has an almond base. Perhaps a few drops of Amaretto in the buttercream would work. That would play nicely with the almond flavor already there.