California Code, Probate Code - PROB § 8403

(a) Before letters are issued, the personal representative shall take and subscribe an oath to perform, according to law, the duties of the office.  The oath may be taken and dated on or after the time the petition for appointment as personal representative is signed, and may be filed with the clerk at any time after the petition is granted.

(b) The oath constitutes an acceptance of the office and shall be attached to or endorsed on the letters.