Thanks pijiang 老師. The volunteer job is actually a favor from the professor to avoid the 60 days gap for OPT. There isn't a formal process of hiring involved.
1. Will I get into trouble if I put down the job without mentioning its a volunteer job?
2. If I don't put down this job, does it mean I am out of status of OPT and my I-485 will be affected? Thanks again.