I have been issued OPT 20 days before I finished my degree (MS in Computer Sciences). Can it be extended for next 17 months.

Thanks,

First of all, you must be able to find a job within 90 days, if you are not employed by 90 days, you need to leave US permanently and won't be able to claim OPT again.

If you are employed, and the job is in the area specified in STEM then you can file extension before you complete the 12 months term.