opt extension should be straight forward. However keep in mind , last year they changed a couple of rules for the employer

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Complete the Form I-983, Training Plan for STEM OPT Students. In this form, you must attest that:
You have enough resources and trained personnel available to appropriately train the student;
The student will not replace a full- or part-time, temporary or permanent U.S. worker; and
Working for you will he
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https://www.uscis.gov/working-united-states/students-and-exchange-visitors/students-and-employment/stem-opt#_Applying_for_a

Thanks fdpower. Yes, I'm aware of new rules.

I am also in similar situation. Looking for information about OPT extension especially people who were unemployed more than 90 days but got extension without any problem. Were you asked any RFE, like copy of W2, paychecks etc?