A rare oxyphosphinidene (Me-OP) has been generated in the triplet ground state through either photolysis (266 nm) or flash-vacuum pyrolysis (FVP, 700 degrees C) of methoxydiazidophosphine MeOP(N-3)(2). Upon ArF laser irradiation (193 nm), an unprecedented isomerization from Me-OP to the long-sought methylphosphinidene oxide (Me-PO) occurs in cryogenic Ne- and N-2-matrices. Alternatively, the latter can be efficiently generated through photolysis (193 nm) or FVP (ca. 700 degrees C) of methylphosphoryl diazide MeP(O)(N-3)(2), in which the elusive nitrene intermediate MeP(O)(N-3)N in the triplet ground state has been also observed by IR (with N-15-labeling) and EPR (vertical bar D/hc vertical bar = 1.545 cm(-1) and vertical bar E/hc vertical bar = 0.003 95 cm(-1)) spectroscopy.