During Chanukah, Al Hanisim is added to each of the tefillos--Shacharis, Musaf (on Rosh Chodesh), Mincha and Ma’ariv. It is added to Birchas Hamozon as well. The following halochos pertain to a person who omitted Al Hanisim: 1) If he remembers prior to saying Hashem’s name at the end of the brocha, he should return to the proper place and say Al Hanisim. 2) If he only remembers after reciting the name of Hashem, he may say it at the end of Shemoneh Esreh before yihyu l’rotzon. In Birchas Hamozon it can be said as a Harachamon followed by the paragraph of B’yemei Matsiyohu. 3) If Al Hanisim was completely forgotten, Shemoneh Esreh and Birchas Hamozon are not repeated.
[שו"ע תרפב, א-ב, ומשנ"ב ד-ה; ביאורים ומוספים דרשו, 6]