If I used buy back OMERS option and contributed to my pension plan, where, in TurboTax, I include this contribution, under RRSP or somewhere else?

Did you get any T-Slip related to OMER pension plan contributions? 

If so, please be sure to follow these steps in TurboTax.

Thank you for choosing TurboTax.

I received Income Tax Receipt for income tax purposes 

In your tax return, you have to enter all tax slips you received. 

As your question is more a tax related question, we suggest you to contact CRA for this question.

Thank you for choosing TurboTax.

Have you found a solution? I'm in the same boat, and have no idea where to input this, or if there is a T4A coming. 

I put mine in as an RRSP and I think it has messed up my return.

I would recommend contacting OMERS directly to see what slip they will be sending you.