Hilchos Sefiras Ha’omer – 8

1. As soon as it is definite nighttime (when three medium stars have come out), it is permissible to count Sefiras Ha’omer,even before praying Ma’ariv, and after Shabbos, even before reciting Havdalah (Mishnah Berurah489:18).

2. Once the time arrived to countSefiras Ha’omer, it is forbidden to eat [a meal] until one has fulfilled the mitzvah of counting [this is true even of the person already prayed Ma’ariv, for otherwise the person would not be allowed to eat on account of Ma’ariv], unless he has someone who will remind him after he eats that he must still count (ibid.).