Pure rape seed oil (RSO), as coded BO100 (BO: Bio-Oil) to distinguish from biodiesel was investigated for a range of intake oxygen levels from 21 to 24%. RSO can have deposit problems in both the fuel injector and piston crown and elevated intake oxygen levels potentially could control these by promoting their oxidation. Increased intake oxygen elevates the peak temperature and this promotes the oxidation of soot and volatile organic compounds. The effect of this on particle mass and on the particle size distribution was investigated using a 6-cylinder 6-liter Perkins Phaser Euro 2 DI diesel engine. The tests were conducted at 47 kW brake power output at 1500 rpm. The particle size distribution was determined from the engine-out exhaust sample using a Dekati microdilution system and nano-SMPS analyzer. The results showed that for air RSO had higher particle mass than diesel and that this mass decreased as the oxygen level was increased. However, the particle size distribution with BO100 RSO with 21% oxygen was different to that of diesel, with a single distribution peak with a peak at 40 nm compared with 80 nm for diesel and with a higher number for RSO for ≺80 nm. The effect of increasing the intake oxygen level was to reduce the size and the number of the particles with the size for the peak number reduced to 25 nm with 22% oxygen and 15 nm at 23% oxygen. However, the accumulation mode particle number was reduced as oxygen increased. Compared with diesel, the accumulation mode number was less than for diesel for sizes above 40 nm for 22% oxygen and above 25 nm for 23% oxygen and was much less than for diesel at all sizes for 24% oxygen. It was considered that the shift in the size distribution to 25 and 15 nm for 22 and 23% oxygen was due to the greater oxidation rate of soot, but an increase in the nucleation mode with 22 and 23% oxygen occurred due to the reduced soot mass and associated reduction in particle coagulation.