Infrared gas phase spectra of ONCN, ONC¹⁵N, ON¹³CN, O¹⁵NCN, ¹⁸ONCN, O¹⁵NC¹⁵N, O¹⁵N¹³CN, ¹⁸ONC¹⁵N and ¹⁸ON¹³CN have been recorded. A general harmonic force field (with six constraints) has been derived based on the assigned fundamental vibration frequencies and checked by calculation of experimental values for centrifugal distortion constants, a Coriolis coupling constant and the inertial defect. The harmonic force field of a fully optimized structure of ONCN as found by the Pulay force method is reported and compared to the experimental force field. No essential disagreements were found.