- Peninei Halakha
One who is uncertain whether he neglected to count one day may continue counting with a berakha, because we only defer to the opinion that one cannot continue counting with a berakha when one is certain that he missed a day.
Similarly, one who forgets to count at night but remembers and counts during the course of the day must count with a berakha on all subsequent nights. Even though some maintain that one does not discharge one’s obligation by counting during the day, we follow those who maintain that, be-di’avad, one who counts during the day fulfills the mitzva.
The status of a young boy who becomes a bar mitzva during the omer period is uncertain. Some poskim maintain that even if the boy takes care to count every day, he cannot continue counting with a berakha, because the days he counted before becoming a bar mitzva are not considered one continuum with the days he counts after reaching adulthood. According to most poskim, however, if the boy takes care to count every day before becoming a bar mitzva, his counting is considered complete, and he may continue counting with a berakha. This is the prevalent custom.
One who converts to Judaism during the omer period counts without a berakha, because he did not count at all before his conversion.
 Terumat Ha-deshen 1:37 states that even though we customarily follow Behag’s opinion, that is true only when one is certain that he forgot to count. In a case of uncertainty, however, we follow the viewpoint of the majority of poskim. The reasoning is that some authorities maintain that sefirat ha-omer is a Torah obligation. Therefore, when an uncertainty arises, one must be stringent; he must continue to count, and do so with a berakha. (Terumat Ha-deshen adds another rationale: Reciting a berakha le-vatala is only a rabbinic prohibition. However, even sa, who in 215:4 leans toward the opinion that reciting a berakha le-vatala is forbidden by Torah law, rules here in 489:8 in accordance with Terumat Ha-deshen’s opinion.)
According to most poskim, even if one remembers to count only during bein ha-shmashot, he may count the remaining nights with a berakha, even though it is doubly uncertain that he has fulfilled his obligation in such a case: First, some authorities maintain that counting during the day is invalid. Second, even if we maintain that one indeed discharges his obligation by counting during the day, it is uncertain whether bein ha-shmashot is part of the day or the beginning of the following night. Nevertheless, since it is not certain that one missed a day in such a case, he can rely on the majority of poskim who maintain that each day is a separate mitzva and continue counting with a berakha. Furthermore, according to Rabbeinu Tam, we define bein ha-shmashot unquestionably as daytime, and sa 261:2 rules this way as well. Though we do not follow this ruling in practice, we may invoke it in a case of uncertainty in order to rule leniently. Some rule stringently, such as Kaf Ha-ĥayim 489:83, but as we stated above, most poskim maintain that one who counts during bein ha-shmashot may continue counting with a berakha. This is also the position of Sho’el U-meishiv, Yabi’a Omer, oĥ 4:43, and Hilkhot Ĥag Be-ĥag 6:7.
 Birkei Yosef, Ĥidushei Ha-Rim, Yabi’a Omer 3:28, and others state that the boy may not continue counting with a berakha after he becomes a bar mitzva. However, most poskim – including Ktav Sofer §99, ahs 489:15, Kaf Ha-ĥayim §94, Har Zvi 2:76, and Or Le-Tziyon 1:95 – maintain that he may continue counting with a berakha, and this is the prevalent custom. They provide several reasons for this. Since even a young boy must count the omer due to the mitzva of ĥinukh (educating one’s children in Torah and mitzvot), his previous counting is significant and can be viewed on the same continuum as his post-bar mitzva counting. Moreover, even after he becomes a bar mitzva, he will still be obligated to count, at the very least, to fulfill the mitzva of ĥinukh. Furthermore, since he has passed the age of twelve, he is considered mufla samukh le-ish (a youth close to adulthood), whose oaths are binding by Torah law. Since he regularly counted the omer, it is as if he took an oath to count, and he is thereby obligated to do so by Torah law. Regarding the “completeness” of the count, since he actually counted, it should certainly be considered complete, even though he wasn’t originally obligated to the same degree that he is now. Even if it is uncertain whether his counting as a minor is significant enough, we already learned that whenever there is an uncertainty, one may continue counting with a berakha.