No - keep some monies in your OA in the event if you think in the forseeable future you may have issues paying your loan.
For second question - yes to a certain extent. You can pay with CPF (aka HDB loan) and thereafter switch to a bank loan. However the switch cannot be made back to HDB loan once you go into a bank loan.
There are pros and cons to HDB loan and Bank Loan which you can find more information online.
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