However, HDB will automatically deduct all your existing CPF OA amount for your downpayment (plus any cash topup if your CPF is not enough to cover the minimum downpayment), and calculate your loan on the balance amount.
Example, if you had $100k in your CPF, and your new HDB flat costs you $200k, HDB will still deduct $100k from your CPF as downpayment and calculate your monthly loan based on the remaining $100k.
And if your monthly loan is calculated to be $500, and your monthly CPF contribution is $1k, HDB will only deduct $500 each month from your CPF. The remaining amount stay in your OA.
Another thing to note, if you are a Singapore citizen and married, and this is your 1st house, there is a CPF subsidy of $30k if you buy resale flat. Also if you buy a resale flat near your parents, the subsidy is $40k. See link