The Rishonim (Medieval Halachic authorities) debate the question of whether a person can fulfill his obligation of Sefirat Ha’omer by listening to the Hazan’s counting. The famous Halachic principle of "Shome’a Ke’one" establishes that a person can fulfill his obligation to recite a certain text by listening to its recitation by somebody else. So long as both parties – the person reciting the text and the listener – have in mind that the listener fulfills his obligation by hearing the recitation, he indeed fulfills the requirement in this fashion. Seemingly, then, if the Hazan has in mind when he counts the Omer that his recitation should fulfill the obligation for those listening, those in the congregation who wish to fulfill the Misva by listening to the Hazan’s counting should be able to do so.
Some Rishonim, however, claimed that Sefirat Ha’omer marks an exception to the rule of "Shome’a Ke’one." The Torah presents the obligation to count the Omer with the expression, "U’sfartem Lachem" ("You shall count for yourselves" – Vayikra 23:15), which suggests that it refers to a personal obligation, like the Misva of Lulab on Sukkot. If so, then one would be required to count personally, just as one must take the Lulab personally. The word "Lachem" ("for yourselves") in this verse emphasizes that each individual must count for himself, rather than hear the counting from somebody else.
Other Rishonim, however, counter that the word "Lachem" means something else entirely. One might have thought that the obligation of Sefirat Ha’omer is cast solely upon Bet Din, who should count the days until Shabuot on behalf of the rest of the nation. The word "Lachem" was therefore added to instruct that each individual bears this obligation. This is not to say, however, that the standard principle of "Shome’a Ke’one" does not apply.
In light of this debate, it is proper for each individual to personally count the Omer, rather than simply listen to the Hazan’s counting. It must be emphasized that the Hazan customarily counts aloud before the congregation not so that the congregation fulfills its obligation through his counting, but rather to inform everybody of which number day they must count that night. Interestingly enough, the custom in Egypt was, at one point, for the congregation to count before the Hazan, in order to avoid the misconception that one can fulfill his obligation through the Hazan’s counting. The Egyptian communities changed their practice, however, because people frequently counted the wrong day. In any event, one must ensure to count the Omer himself, rather than rely on the Hazan’s counting.
Summary: One must count the Omer personally; the Hazan customarily counts the Omer aloud before the congregation only to inform them of which number day to count, and not for them to fulfill their obligation by listening to his counting.
