If my OA is sufficient for down payment for BTO, is it better to invest the cash flow reserved for down payment now or continue saving and pay the debt when buying BTO?

If the cash flow is reserved just for your BTO downpayment (not for the reno etc), you can then DCA into investments!

Another way looking at this question is whether you want to use cash/CPF OA to downpay for your BTO?

Have you set aside emergency fund?

When do you need to make the down payment?

What do you intend to invest with your cash?

Your down payment is secured with your OA. If you have also set aside emergency fund in cash, then you can consider investing your cash flow reserved for down payment to achieve better returns than leaving in the bank.