I have just transferred my car to an agent from motorist.sg. It is an OPC. Since turning OPC I've been getting a rebate of $1.1k every 6 months from LTA. Will I still get my OPC cash rebate for the last 6 months?

No, you will not get anymore rebates since the car is no longer under your name. Once you have sold your car, the rebates will then go to the new owner of the car, and in your case, it will go to the dealer who bought the car from you.