  The CW microcavity optical parametric oscillator (OPO) state is investigated
using a theoretical treatment which includes the contributions of the pump,
signal and idler populations to the renormalisation of the polariton energies
(the `blue-shift'). The theory predicts the pumping conditions under which the
OPO switches on, showing that a pump angle > 10 degrees is required, but there
is no particular significance to the `magic-angle' where pump, signal and idler
are all on resonance. The signal and idler renormalisation contributions also
causes the signal emission to be driven towards the normal direction as the
pump power increases above threshold.
