Economic development of many of the world's oil and gas reserves will require the application of improved drilling methods such as underbalanced drilling (UBD), a technique used to minimize formation damage in hydrocarbon reservoirs. In UBD operations, oxygen-reduced air is often injected along with an oil-based drilling fluid. This oxygen-reduced air may contain up to 5% oxygen which can react with oil and/or an oil-based drilling fluid during the drilling process to form potentially hazardous oxidized hydrocarbons. This could lead to the presence of potentially flammable or explosive mixtures in the surface piping and/or drill string, particularly the return annulus. Discussion of the oxidation behaviour of light oils and oil-based drilling fluids in the literature is limited, particularly regarding the effects of long-term exposure to oxygen. This paper characterizes the oxidation behaviour of two light oils and an oil-based drilling fluid using thermogravimetry (TG) and differential scanning calorimetry at 89 and 9,811 kPa (1,4 10 psig). The effects of core, pressure, and pre-exposure to oxygen on subsequent oxidation behaviour were studied. The results show that the addition of crushed core enhanced the oxidation reactions of the oil samples and delayed exothermic activity of the drilling fluid. Elevated pressure accelerated the oxidation reactions. Pre-exposure to oxygen was shown to increase the reactivity of the oils and drilling fluid during subsequent oxidation. The results indicate that under certain conditions, oxidation processes could begin at significantly reduced temperatures, possibly leading to ignition of the hydrocarbon mixtures.