Technically you should be able to file for PERM/GC while on OPT. Applying for OPT on the otherhand while LC is pending would have been risky as F1 (OPT) has a pure non-immigrant intent. As you are already on OPT, filing for LC and later on for H1B which is a dual intent status shouldn't be a problem. Only concern will be maintaining your status during the Cap Gap and questions you might face during your visa interview should you need to go out of US for your H1B stamping. It's better to consult your lawyer and discuss all the scenarios.