Once you renew your COE, you will only receive road tax rebates and no longer be entitled to the OPC rebate. The OPC rebate is a one time rebate on the first purchase of COE. Thereafter it does not apply.

Here's more on the things to take into consideration before you renew your car's COE: http://www.motorist.sg/article/120/renew-coe-vs-buying-a-car-which-is-really-better-for-you