Personnly I think, whenver you submit your F1 to H4 application(e.g. Nov20) before OPT expired date(Dec 1st),you are legal. the waiting period (after Dec 1) is called Pending period. you are still legal status.
In fact,as our case, from Aug 28 (School Starts Date) to Sept 4, we don't belong to neither F1 nor H4.it is pending status.
but I don't know if you submit your application after OPT expired...consult the lawyer or others can give you more help.
GOOD LUCK