  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), observed
with high signal-to-noise and high spectral resolution with the VLT-ISAAC
spectrometer, providing the first opportunity to study the solid OCN$^-$
abundance toward a large number of low-mass YSO's. It is shown unequivocally
that 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 the
2165.7-component can be ascribed to OCN$^-$, embedded in a strongly
hydrogen-bonding, and possibly thermally annealed, ice environment based on
laboratory OCN$^-$ spectra. In order to correct for the contribution of the
2175.4-component to the XCN band, a phenomenological decomposition into the
2165.7- and the 2175.4-components is used to fit the full band profile and
derive the OCN$^-$ abundance for each line-of-sight. The same analysis is
performed for 5 high-mass YSO's taken from the ISO-SWS data archive. Inferred
OCN$^-$ 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 at
least a factor of 10--20 and large source-to-source abundance variations are
observed within the same star-forming cloud complex on scales down to 400 AU,
indicating that the OCN$^-$ formation mechanism is sensitive to local
conditions. The inferred abundances allow quantitatively for photochemical
formation of OCN$^-$, but the large abundance variations are not easily
explained in this scenario unless local radiation sources or special geometries
are invoked. Surface chemistry should therefore be considered as an alternative
formation mechanism.
