In this work, secondary ion mass spectroscopy was used to study carbon and oxygen impurity incorporation in N-polar [000-1] GaN films grown by MOCVD. The effects of growth temperature, V/III ratio, and precursor flows were studied within a regime relevant to low temperature (In,Ga)N:Mg growth for device structures containing high indium concentrations. For films grown without magnesium, oxygen levels were between 3 and 8x10(16) cm(-3) and did not depend strongly on the growth conditions. Mg-doped films yielded higher oxygen concentrations ranging from 8x10(16) to 4x10(17) cm(-3) depending on the gallium and magnesium precursor flow rates. The carbon concentration in the films varied significantly with different growth conditions, and was most affected by the growth temperature and the V/III ratio. With careful tuning of the growth parameters carbon and oxygen concentrations of 2 and 4x10(16) cm(-3), respectively, were achievable under a wide range of conditions.