yep, that's right. OPT means you can work only on your field. grocery store, gas station, etc ma kaam garnu illegal ho. you can take the risk if you want, but in case thaha payo bhane ghar pathaidinchha. tyahi bhayera ma chahi gas station ma kaam garera risk naliu bhanchhu. but, good thing is there is no hour limits in OPT, you can work 100 hours a week if you want and you can have multiple jobs related to your field. idk what kind of engineering associate degree you have but let's say your OPT primarily consists of  working in some sort of physics research lab. on the side, you can be a TA for some physics class in the same institution, (may be in any other institution as well, but i'm not quite sure about that).