For a property with remaining lease that is unable to cover the youngest buyer using CPF until age 95, you and your co-owners (if any) can use your respective OA savings up to a percentage of the lower of the purchase price or the valuation price of the property at the time of purchase.
When the total CPF usage for all owners reaches this pro-rated limit, you will not be able to use further OA savings for the property, regardless of whether you have set aside your Basic Retirement Sum.