No, it's not, and my DD-256 separating me from the Reserves after my 4-year active-duty hitch specifically relieved me of duty to my oath. Needless to say, I agree with our late friend on an emotional level, but legally, no, once separated, an enlisted troop is released from obligation.
Officers, on the other hand, while released from their oaths, may be called back into commission and prosecuted under UCMJ authority for crimes committed after their service.