The pure rotational far infrared absorption spectrum of nitrosyl cyanide, ONCN, is reported in the spectral range 20-100 cm^-1. The spectrum shows a characteristic K-sub-band structure yielding the values of (A - C̄) = [A - (B + C)/2], D_K and H_K for both the ground state and the two vibrationally excited states, v_s = 1 and v₆ = 1. In the region 180-320 cm^-1 the vibration-rotation bands v₅= 0 → 1 and v₆ = 0 → 1 are observed for ONCN and O¹⁵NCN, also with pronounced K-subband structure. An analysis yields v₅ = 212.0 cm^-1 and v₆ = 264.2 cm^-1 for ONCN and v₅ = 211.6 cm^-1 and v₆ = 264.0 cm^-1 for O¹⁵NCN. The Coriolis coupling constant ζ₅₆ is estimated to be 0.57.