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The 4.62$\mu$m (2164.5 cm$^{-1}$) `XCN' band has been detected in the$M$-band spectra of 34 deeply embedded young stellar objects (YSO's), observedwith high signal-to-noise and high spectral resolution with the VLT-ISAACspectrometer, providing the first opportunity to study the solid OCN$^-$abundance toward a large number of low-mass YSO's. It is shown unequivocallythat at least two components, centred at 2165.7 cm$^{-1}$ (FWHM = 26 cm$^{-1}$)and 2175.4 cm$^{-1}$ (FWHM = 15 cm$^{-1}$), underlie the XCN band. Only the2165.7-component can be ascribed to OCN$^-$, embedded in a stronglyhydrogen-bonding, and possibly thermally annealed, ice environment based onlaboratory OCN$^-$ spectra. In order to correct for the contribution of the2175.4-component to the XCN band, a phenomenological decomposition into the2165.7- and the 2175.4-components is used to fit the full band profile andderive the OCN$^-$ abundance for each line-of-sight. The same analysis isperformed for 5 high-mass YSO's taken from the ISO-SWS data archive. InferredOCN$^-$ abundances are $\leq$ 0.85 % toward low-mass YSO's and $\leq$ 1 %toward high-mass YSO's, except for W33 A. Abundances are found to vary by atleast a factor of 10--20 and large source-to-source abundance variations areobserved within the same star-forming cloud complex on scales down to 400 AU,indicating that the OCN$^-$ formation mechanism is sensitive to localconditions. The inferred abundances allow quantitatively for photochemicalformation of OCN$^-$, but the large abundance variations are not easilyexplained in this scenario unless local radiation sources or special geometriesare invoked. Surface chemistry should therefore be considered as an alternativeformation mechanism

A 3–5 $\mathsf{\mu}$m VLT spectroscopic survey of embedded young low mass stars II