Question: I see that many people say leshem yichud before counting the omer yet we didn’t do so in yeshiva. Should I begin saying it?
Answer: Many have a minhag to say the kabbalistic prayer, leshem yichud, before performing certain mitzvos, expressing one’s intentions to fulfil the mitzva to serve Hashem properly in order to properly focus and prepare themselves in advance. Thus, the Chida (Moreh Baetzba 1) and Yesod Veshoresh Ha’avoda (9:8) stress the importance of this mitzva, and write that one must channel their thoughts and emotions to prepare themselves for this mitzva by saying leshem yichud (See Minhag Yisrael Torah 8:1).
Nonetheless, this practice, while printed in most siddurim, is a most controversial one. R’ Yechezkel Landau (Noda Biyehuda YD 93) wrote very strongly against saying leshem yichud, trying to get it removed from the siddur.
This debate aside, other poskim point to particular textual issues within the leshem yichud preceding the sefira.
While Rambam (Temidin Umusafin 7:22) and the Sefer Hachinuch (306) hold that the mitzva to count the omer nowadays is mideoraisa, most poskim (Tosafos, Menachos 66a; Rosh, Pesachim 10:40; Ran, end of Pesachim) hold that it is miderabanan (See Mishna Berura 489:14). Thus, R’ Yaakov Kamenetsky (Emes L’yaakov OC 489) and R’ Moshe Sternbuch (Teshuvos Vehanhagos 2:247) challenge how we can say, ‘כמו שכתוב בתורה, as it is written in the Torah.’ Doing so is tantamount to bal tosif, adding onto the Torah’s mitzvos. They propose amending the text slightly.
Nonetheless, R’ Shlomo Zalman Auerbach (Halichos Shlomo, Sefiras Haomer 11:2) defends saying this text, writing that this isn’t a problem of bal tosif as we are obligated to count miderabanan.
The Aruch Hashulchan (OC 489:10) writes that while many have challenged saying these Kabbalistic prayers, it has become common practice to say them.In conclusion, there are strongly held minhagim on each side with reasons both to say it and omit it.