Oblique detonation waves (ODWs) in stoichiometric acetylene-oxygen mixtures, highly diluted by 81-90% argon, are studied using the reactive Euler equations with a detailed chemistry model. Numerical results show that the incident Mach number M-0 changes the ODW initiation structure, giving both the smooth transition in the case of M-0 = 10 and the abrupt transition in the case of M-0 = 7. By comparing results of numerical simulation and theoretical analysis, the initiation processes are found to be chemical kinetics-controlled regardless of M-0, different from those in hydrogen-air mixtures which are wave-controlled in the low M-0 regime. The argon dilution effect on the initiation morphology is investigated, showing that the structures are determined by the dilution ratio and M-0 collectively. However, the initiation length is found to be independent of the dilution ratio and only determined by M-0, which is attributed to the competing effect of the high density and high temperature. (C) 2019 The Combustion Institute. Published by Elsevier Inc. All rights reserved.