OPT is indeed one form of employment authorization with some restrictions. If you are already on OPT, you need to include copies of your OPT card and receipts and needless to mention both sides of I94 to show your current legal status (F1 and then OPT). If you have not yet received your OPT, you better include just copies of your NOAs and a cover letter explaining the situation.
Keep in mind that extra documentation wont hurt you, but missing documents could cause lots of delays and problems.