I used my Home buyer plan for a home purchase that closed July 2018. Can I use the contributions to my PRPP (OMERS) from Aug 2018-Dec 2018 to repay my HBP?

Yes, you can use contributions to your RRSP, Pooled registered pension plan (PRPP) or Specified pension plan (SPP) to repay your Home Buyers Plan (HBP). 

You can choose to make repayments before the repayment period starts, here's how CRA treats those early repayments.

Your repayment period starts the second year after the year you withdrew funds from your RRSP(s) for the HBP,  so if you choose to start your repayments earlier, your repayment period will stay the same. Any repayments made before you are required to start your repayments will reduce the amount you have to repay for the first year. If your early repayment(s) are  more  than the required amount for the first year, the difference will reduce your HBP balance and your remaining repayment amounts over the entire repayment period.

See this link with the CRA information on repaying the HBP: https://bit.ly/2RGRh4X