Vanadium samples with controlled oxygen impurity content (6-2460 at.ppm) were irradiated with fast neutrons to a fluence of 8.3x10(18) n/cm(2) at about 100 degreesC. Effects of oxygen impurities on the formation of irradiation-induced defects and the interaction between the defects and oxygen impurities are studied systematically by combining the position lifetime and coincidence Doppler broadening (CDB) techniques. It is found that the lifetimes longer than 380 ps are due to positron trapped at irradiation-induced microvoids. From the CDB measurement, the fingerprints of oxygen-impurity decoration of the defects am found in the high momentum region.