Don't know about the tach, generally they work as the wiring is all there. Check all your connections on the multipoint plug(s) at the back of the cluster.
In regard to the oil pressure gauge, have you changed the oil pressure switch on the engine block for an oil sender unit? This must be done to make the gauge work. Pays to grab a sender unit when you buy the "S" pack cluster.
get hold of another cluster with a tacho ( take it apart completely and take the tacho unit out and swap it over.....it isnt too hard, all the gauges come out as separate units and just remember where all the individual screws came from when taking ur good one apart and u should be right..