This matter has been debated by the Acharonim, and it is worthwhile to understand the various, logical arguments and their relative strengths to help decide how to act.
We will start with the assumption that after missing a day of sefirat ha’omer, one may not continue counting with a beracha, because the forty-nine days constitute one mitzva of counting.[1] The Pri Chadash[2] offers the following, tentative suggestion. Although the beracha is inappropriate personally for one who has missed, it is proper for him to recite it as the congregation’s representative for the public recital of sefirat ha’omer. The public mitzva, even if everyone present is capable of performing the mitzva himself, creates an obligation on the public level, as it does regarding chazarat hashatz. Alternatively, the Beit HaLevi reportedly[3] suggested that someone in the minyan can be asked to refrain from making his own beracha and be yotzei with the person who forgot a day. That way, the beracha becomes appropriate based on the rule that one can make a beracha on someone else’s behalf even if the person making the beracha is not doing the mitzva himself at that time.[4]
The Pri Chadash[5] goes on to reject these possibilities because the person who forgot a day, assuming, as above, that he is unable to fulfill the mitzva of sefirat ha’omer, is akin to one who is not obligated in the mitzva. Such a person is incapable of making the beracha to be motzi someone else.[6] He reasons that although the person in question is generally obligated in sefirat ha’omer, the fact that he has no practical obligation at this time makes him equivalent to the person in the following case of the Yerushalmi. The Yerushalmi says that one whose obligation to read Megillat Esther is on the fourteenth of Adar cannot read on behalf of those who are obligated on the fifteenth of Adar.
The Birkei Yosef[7] cites (but rejects) those who take issue with the Pri Chadash’s comparison, as follows. In the case of megilla, the person in question has no obligation to read on that day. In contrast, our chazan is obligated today and just a technical (halachic) impediment prevents him from fulfilling the mitzva. Rav Frank[8] points out that the Talmud Bavli posits that the responsibility to help another Jew fulfill his mitzva (arvut) makes it considered as if he has a personal obligation. Thus, he reasons, the Bavli must reject the aforementioned Yerushalmi. Although there is not a clear conclusion on the matter, the majority opinion seems to follow the Pri Chadash, that the person who missed a day of sefirat ha’omer should not use the Beit HaLevi’s trick to enable him to make the beracha.[9] On the contrary, he should have in mind to be yotzei with one who has not missed a day.
One very subjective, pertinent factor is the element of embarrassment. K’vod haberiyot (avoiding embarrassing people, including oneself) has great, halachic weight. Thus, there are those who allow a rav who customarily does the sefira aloud and for whom it would be a disgrace to publicize that he missed a day of sefirat ha’omer to rely on the very significant opinions among Rishonim that missing one day of sefirat ha’omer does not disqualify the mitzva thereafter.[10] Someone other than the rav should probably not be so embarrassed in such a situation, and the rav can pasken for himself. Therefore, our suggestion is that a regular chazan should preferably offer to someone else the honor of saying the sefira aloud or perhaps avoid being the chazan during that period if he will be embarrassed. (Regarding mourners, they usually do not miss days anyway.)