Partial hdb loan repayment: You reduce the 2.6% p.a. interest and reduce new occasions for the phantom 2.5% p.a. interest to build up.
Refund to CPF OA: You reduce the phantom 2.5% p.a. interest that will materialise when you sell your unit but the 2.6% p.a. interest from your loan still rolls.
Partial hdb loan repayment: You reduce the 2.6% p.a. interest and reduce new occasions for the phantom 2.5% p.a. interest to build up.
Refund to CPF OA: You reduce the phantom 2.5% p.a. interest that will materialise when you sell your unit but the 2.6% p.a. interest from your loan still rolls.
The first option makes more sense to me.