Yes, you can enter P O Box as your address only if the post office does not delivermail to your physical address.
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Please note: This advice is provided with the understanding that all the relevant facts have been provided by you. Any change in facts might affect the advice given and hence may not be relied on in such cases.
Note that you can use P O Box only if the post office wont deliver to your physical address. So you cannot hide your physical address if you have one. Infact IRS has the information on your physical address through the mortgage interest, W-2 , social security and other forms filed with the IRS.