Do not worry.
Here are the answers to your questions:
1. Can I use the EAD to work for others if the company reports my layoff to INS?
YES. You MUST have EAD to work for other employer(s) when your I-485 application is pending.
2. Because my H1b visa will expire soon and I cannot renew anytime, Can I apply for O-visa during the period of EAD working?
No need to apply for O visa as you are in valid "adjustment applicant status". Just file for your EAD and start looking for work. You can start working on the new after EAD approval.
3. Any advice how I can go through this situation?
I don\'t think that there is anything to worry about. Just look for new job in same or similar occupation with SAME job resonsibilites.
In case of employment RFE, send a letter from your new employer stating your job title, job desc, salary, and job location. This letter must state that this is a permanent, full time job.
I wish you best of luck.
* Use the info at your risk. I am NOT a lawyer *