Turbulence-chemistry interaction of inverse gaseous oxygen/hydrogen coaxial jet flames in a test model combustor at elevated and high pressures (up to 23.6 bar) has been studied experimentally. Since it is difficult to experimentally determine the Damkohler number (Da), an important parameter representing the turbulence-chemistry interaction, for turbulent nonpremixed flames, a new method to estimate Da using the flame visualization by OH planer laser induced fluorescence and OH* chemiluminescence is suggested. The proposed modified Da that is based on the flame base location where the maximum OH* intensity is observed and the mean OH layer thickness provides the limits for the occurrence of combustion instability leading to local flame extinction. Thus, it is found to be useful for representing the regimes that involve the local flame extinction, particularly when considering a wide range of pressure conditions where thermochemical and transport properties simultaneously vary and thus complicated phenomena are observed. The combustion instability is observed only for Da <= 0.4, and it is found that the OH layer thickness generally decreases, the disconnected OH reaction zone is reduced, the occurrence of the local flame extinction is suppressed and the maximum value of flame surface density is enhanced with increasing combustion pressure. (C) 2019 Elsevier Ltd. All rights reserved.