The comparability tab for OCC will tell you whether a sample is coded using ISCO 1988, 2008, or using a separate method. For instance, both Armenia and Belarus use the ISCO 1988 classification and are generally comparable with each other. In cases where the same coding scheme is used, but the number of digits differ, you may add zeros to make the values comparable. However, this will result in the same occupations having different codes. Two-digit occupation codes do not provide the same level of detail as three-digit codes and therefore it’s unclear which three-digit code individuals with two-digit codes should map to. My recommendation would be to instead remove a digit from three-digit codes in order to run your analysis on this sample. Alternatively, you may use the general recoded variable, OCCISCO, which uses a single-digit ISCO 1988 scheme.
This forum discussion may also be helpful.