The Shulhan Aruch writes amidst his discussion of Sefirat Haomer that during the Omer period, one may not sit down to a large meal within a half-hour of nightfall, the point at which he becomes obligated to count. Thus, for example, if the time for counting begins at 8:30 pm, one may not sit down to a large meal from 8:00 pm until he counts the Omer. The Rabbis were concerned that if one begins a large meal within a half-hour of the time when he must count, his involvement in the meal may cause him to forget to count.
This Halacha applies only to a meal consisting of 2 oz. or more of bread. One may eat other foods or lesser amounts of bread within a half-hour of the time of Sefirat Haomer.
The Ben Ish Hai (Rabbi Yosef Haim of Baghdad, 1833-1909), in his commentary to the Haggada, writes that it is possible to circumvent this Halacha by appointing a "Shomer" (literally, "watchman"), somebody to remind him to count the Omer. If a person designates somebody to remind him about the obligation to count, he may then sit down to a large meal within a half-hour of the time for counting. However, as the Ben Ish Hai emphasizes, this Halacha is subject to a number of conditions. Firstly, the "Shomer" cannot be eating a meal at the same time. If the "Shomer" also eats a meal, then we must be concerned that he will also forget to count the Omer, and his designation thus does not help ensure that the individual who appointed him will remember to count. Furthermore, the "Shomer" must be somebody who is obligated to count the Omer; otherwise, the "Shomer" is not thinking about the counting and therefore cannot be relied upon for a reminder. Therefore, a woman cannot serve as a "Shomer," since women are not obligated in the Misva of Sefirat Haomer.
The Ben Ish Hai raises the interesting question of whether a person who had missed a complete day of counting may serve as a "Shomer." If one missed an entire night and day of counting, he continues counting the Omer but without a Beracha. The question thus arises as to whether or not we consider him somebody obligated to count, such that he is qualified to serve as a "Shomer." The Ben Ish Hai concludes that such a person may, in fact, serve as a "Shomer," since he is, after all, required to count the Omer. Some authorities maintain that each day of the Omer count constitutes a separate obligation, and therefore one who misses a day must still continue to count. For this reason, one who missed a day should continue counting, albeit without a Beracha. Since he must continue counting, the obligation is indeed on his mind, and he may therefore serve as a "Shomer" appointed to remind somebody to count after his meal.
Summary: Within a half-hour of the time for Sefirat Haomer, one may not begin a meal in which he will eat 2 oz. or more of bread, unless he first appoints a man to remind him to count the Omer when the time arrives. That man whom he appoints must be somebody who is not eating a large meal at that point. It does not suffice to appoint a woman to remind one to count the Omer, since women are not included in the obligation of Sefirat Haomer.
