For the reaction of O(P-3) with propyne, the product channels and mechanisms are investigated both theoretically and experimentally. Theoretically, the CCSD(T)//B3LYP/6-311G(d,p) level of calculations are performed for both the triplet and singlet potential energy surfaces and the minimum energy crossing point between the two surfaces are located with the Newton-Lagrange method. The theoretical calculations show that the reaction occurs dominantly via the O-addition rather than the H-abstraction mechanism. The reaction starts with the O-addition to either of the triple bond carbon atoms forming triplet ketocarbene (CH3CCHO)-C-3 or (CH3COCH)-C-3 which can undergo decomposition, H-atom migration or intersystem crossing from which a variety of channels are open, including the adiabatic channels of CH3CCO + H (CH2CCHO + H), CH3 + HCCO, CH2CH + HCO, CH2CO + CH2, CH3CH + CO, and the nonadiabatic channels of C2H4 + CO, C2H2 + H-2 + CO, H-2 + H2CCCO. Experimentally, the CO channel is investigated with TR-FTIR emission spectroscopy. A complete detection of the CO product at each vibrationally excited level up to v = 5 is fulfilled, from which the vibrational energy disposal of CO is determined and found to consist with the statistical partition of the singlet C2H4 + CO channel, but not with the triplet CH3CH + CO channel. In combination with the present calculation results, it is concluded that CO arises mainly from the singlet methylketene ((CH3CHCO)-C-1) dissociation following the intersystem crossing of the triplet ketocarbene adduct ((CH3CCHO)-C-3). Fast intersystem crossing via the minimum energy crossing point of the triplet and singlet surfaces is shown to play significant roles resulting into nonadiabatic pathways for this reaction. Moreover, other interesting questions are explored as to the site selectivity of 0(3 P) atom being added to which carbon atom of the triple bond and different types of internal H-atom migrations including 1,2-H shift, 3,2-H shift, and 3,1-H shift involved in the reaction.