Oxygen-fuel-diluent mixing is important for oxy-combustion as it impacts emissions, efficiency (oxygen slippage) and pattern factor. In this work, rapid mixing validation testing was conducted. Combustion testing was performed with natural gas and syngas from atmospheric pressure (1 atm.) to high pressure (10 atm.) to demonstrate stable operation. Oxy-natural gas testing demonstrated stable operation and low CO emissions over a wide range of test conditions. Syngas testing showed low CO emissions and a wider operability range with high dilution/low oxygen levels. Complete oxygen burnout was accomplished, indicating no oxygen slippage occurred at stoichiometric conditions and hence improved cycle efficiency.