Hybrid density functional calculations are used to study geometries together with H-1, C-13, N-14 and O-17 isotropic hyperfine coupling constants for the 3-oxabutan-2-iminoxy radical. The effect of 12 different basis sets on the calculated hyperfine parameters is investigated. Particular emphasis is placed on the performance of the B3LYP functional in the description of the unpaired electron interaction with the iminoxy O-17 nucleus. It is found that the EPR-III basis sets of Barone and TZVP of Godboud and Andzelm lead to the best quantitative agreement between theoretical and experimental hyperfine constants; the mean absolute deviation from experiment for C-13, N-14 and O-17 is ca. 2 G.