High altitude, lung disease and carbon monoxide poisoning are all known to cause hypoxia. High altitude continues to be a threat to the safety and operations of pilots and mountaineers. Hypoxic exposure can affect various functions in the human body, particularly cognition and the cardiovascular system. However, there is an uncertainty surrounding the effect hypoxia may have on both cognition and the cardiovascular system. The objectives of the study were to examine cognition using the Wechsler Abbreviated Scale of Intelligence – First Edition (WASI-I), evaluate brain tissue oxygenation levels using near-infrared spectroscopy (NIRS) and investigate cardiovascular parameters using heart rate variability (HRV) under different O2 concentrations of 12%, 16.5% and 21%. Seventeen healthy volunteers (nine males and eight females) with a mean (± SD) age of 21.11 ± 1.21 years and BMI of 22.51 ± 2.58 kg m-2 participated in two tasks of the WASI-I; block design (BD) and matrix reasoning (MR). BD, MR and HRV were measured during the three different gas interventions; which were randomised. HRV data was also normalised by considering the heart rate. A subset of ten subjects had their cerebral tissue oxygenation levels (TOI) evaluated using NIRS. Cognitive testing under different O2 concentrations for BD (p=0.133) and MR (p=0.237) were not significantly different. HRV data showed a decrease in the non-normalised high frequency (HFnu) for MR subset (p=0.001) with decreasing O2 concentration. Mean heart rate for BD (p=0.016) and MR (p=0.007) also significantly declined with decreasing O2 concentration. NIRS data showed the mean TOI did not significantly change (p=0.611) during BD, however during MR, TOI (p=0.003) significantly decreased with lowering O2 concentration. The cognitive tests did show decreasing trends, albeit non-significant, with increasing severity of hypoxia. However, parasympathetic activity and cerebral tissue oxygenation both fell during MR with increasing hypoxia. HR also decreased during hypoxia for both BD and MR. Data from this study can benefit patients who suffer from hypoxia, athletes, and persons at high altitude or in industry.