The gas-phase reaction of ozone with alkenes is known to be a dark source of HOchi radicals (such as OH, H, and R) in the troposphere, though the reaction mechanism is currently under debate. It is understood that a key intermediate in the reaction is the carbonyl oxide, which is formed with an excess of vibrational energy, The branching ratios of the ozone-alkene reaction products (and thus HOchi yields) depend critically on the fate of this intermediate: it may undergo unimolecular reaction (forming either OH or dioxirane) or be collisionally stabilized by the bath gas. To investigate this competition between reaction and quenching, we present direct, pressure-dependent measurements of hydroxyl radical (OH) yields for a number of gas-phase ozone-alkene reactions. Experiments are carried out in a high-pressure flow system (HPFS) equipped to detect OH using laser-induced fluorescence (LIF), Hydroxyl radicals are measured in steady state, formed from the ozone-alkene reaction and lost to reaction with the alkene. Short reaction times (usually similar to 10 ms) ensure negligible interference from secondary and heterogeneous reactions. For all substituted alkenes covered in this study, low-pressure yields are large but decrease rapidly with pressure, resulting in yields at 1 atm which are significantly lower than current recommendations and indicating the important role of collisional stabilization in determining OH yield. The influence of alkene size and degree of substitution on pressure-dependent yield is consistent with the influence of collisional stabilization as well as the accepted reaction mechanism.