A biogas fuelled constant speed spark ignition engine was studied experimentally for its performance, emissions and combustion, under the influence of an increased oxygen concentration in the intake air and results were compared. A single cylinder diesel engine was modified for the purpose and was operated at 1500 rpm, maintaining the throttle opening at 25% and 100% for various equivalence ratios. The oxygen level in the intake air was kept at 21%, 22% and 23% by volume and the tests also maintained a compression ratio of 13:1 with a masked valve. A significant improvement in the brake thermal efficiency and brake power was observed at higher oxygen levels. The peak brake thermal efficiencies with 22% and 23% oxygen levels are 27% and 28% respectively, whereas with 21% oxygen level at the same equivalence ratio the efficiency to be 26.2%. The lean limit also got extended and at higher oxygen levels increased NOx, reduced HC and CO emissions were measured. Heat release rates showed enhanced combustion rates, which in turn were indicators for improvised thermal efficiencies. To maintain the NOx emissions well inside the set standards, a mere increase of 1%–2% oxygen level was observed to be ideal. © 2017 Elsevier Ltd