It is customary each night during the Omer period for the Chazan to recite the Beracha and count the Omer aloud, before the congregation recites the Beracha and counts. This practice developed due to the concern that people may mistakenly count the wrong number, in which case they would not fulfill the Mitzva and would recite a Beracha Le'vatala ("wasted" Beracha). As it was assumed that Chazanim would know the correct number for the counting, the custom developed that the Chazan would first count aloud before the congregation. This practice is mentioned already by the Rashba (Rabbi Shlomo Ben Aderet, Spain, 1235-1310), in one of his responsa (126), and is discussed as well by Rabbi Chayim Palachi (Turkey, 1788-1868) in his work Mo'ed Le'kol Chai (5:19).
Rabbi Shneur Zalman of Liadi (the "Ba'al Ha'tanya," Russia, 1745-1813), in his code of Halacha (Shulchan Aruch Ha'Rav, 489:12; listen to audio for precise citation), notes an interesting Halachic dilemma that results from this custom. There is a debate among the authorities as to whether a person fulfills a Mitzva by performing the given action without intending to satisfy his Mitzva obligation ("Mitzvot Tzerichot Kavana" or "Mitzvot Einan Tzerichot Kavana"). According to one view, a person fulfills his obligation regardless of whether or not he has the Mitzva in mind while performing the given act. If so, Rabbi Shneur Zalman notes, once the Chazan publicly counts the Omer, everybody who hears his counting has fulfilled their obligation of Sefirat Ha'omer. Even though they did not intend to fulfill their obligation by listening to the Chazan's counting, and the Chazan did not have in mind for his counting to fulfill their obligation, they nevertheless fulfill the Mitzva by hearing his counting. Rabbi Sheur Zalman adds that with regard to Mitzvot De'Rabbanan (obligations ordained by the Rabbis, as opposed to Torah law), all authorities agree that intention is not indispensable for the fulfillment of the Mitzva. Therefore, since we generally follow the view that Sefirat Ha'omer nowadays is required on the level of Rabbinic obligation, according to all opinions one fulfills his obligation to count by hearing the Chazan's counting. Seemingly, then, it should be forbidden to count with a Beracha after one hears the Chazan count the Omer. Since one fulfills his obligation by listening to the Chazan, even if he did not intend to fulfill his obligation in this manner, he may no longer count with a Beracha.
Chacham Ovadia Yosef addresses this issue in his work Chazon Ovadia (Laws of Yom Tov, p. 228). He writes that in order to avoid this dilemma, one should verbally declare each year towards the beginning of the Sefira period that throughout the period he does not intend to fulfill his obligation by listening to somebody else's counting. By making such a declaration, one establishes that he intends on fulfilling his obligation only by personally counting the Omer, and he may thus count with a Beracha even after listening to the Chazan's counting.
Summary: It is customary for the Chazan to count the Omer aloud with the Beracha before the congregation counts. One should verbally declare at the beginning of the Sefira period that throughout the Omer he intends to fulfill his obligation to count only through his personal counting, and not by listening to the Chazan or other person's counting.
