The מחבר (סימן תפ”ט סעיף ב’) brings the halacha of counting sefiras haomer. Even though להלכה we pasken that today the mitzvah of sefiras haomer is only מדרבנן, there are many Rishonim who hold that it is still דאורייתא. Therefore the ביאור הלכה says that it is best to count at צאת הכוכבים. Women are not obligated in this mitzvah since it is a מצוה שהזמן גרמא. The מגן אברהם holds that women accepted upon themselves as an obligation to count sefiras haomer. According to the מגן אברהם they may count with a brocha. The Mishnah Berura says the minhag is that women do not count and if they want to count they should do so without brocha. According to the Zohar and the Arizal, women should not count at all. However in many homes the women do count with a brocha. This was the minhag in the home of the Steipler zt”l and he would always remind his wife to count.
The מחבר (סימן תפ”ט סעיף א’) says ומצוה על כל אחד לספר לעצמו – there is a mitzvah on every individual from Klal Yisroel to count sefiras haomer. The Mishna Berura brings a machlokes haposkim if there is a halacha of שומע כעונה by sefiras haomer. The Torah says וספרתם לכם that it is a mitzvah on the individual to count. It seems that the mitzvah of sefirah is different than other mitzvos that are done by speaking, for example Kiddush and havdalah where one can fulfill his mitzvah through שומע כעונה by hearing it from someone else. When it comes to sefirah however the Torah says that every individual should count themselves, and שומע כעונה does not work. Others however understand that the Torah is not telling us that everyone must count themselves and not through שומע כעונה, rather that the mitzvah is not on בית דין like counting שמיטה ויובלות. The obligation is on every יחיד, however a יחיד can fulfill his mitzvah through שומע כעונה. The ביאור הלכה rules הלכה למעשה that לכתחילה everyone should count themselves, and בדיעבד if one counted through שומע כעונה he should count again that night without a brocha.
This difference of opinion is only in regard to the actual counting, however everyone agrees that one can use שומע כעונה for the brocha of sefirah. Nevertheless the Mishna Berura says that the minhag is for everyone to make his own brocha and not be יוצא with the brocha of the שליח ציבור.
Still the שער הציון (ס”ק ה’) says that if one is in a situation of a ספק where he is not able to count with a brocha, he should still try to hear the brocha from someone else. The שער הציון adds ויחשוב בדעתו לצאת בהברכה אם הוא מחויב בדבר – he should think to himself that he only wants to be יוצא with this brocha if he still has an obligation to count. This seems to be a very big חידוש. It seems from the שער הציון that if he does not have this מחשבה it may be a ברכה לבטלה. Why? Isn’t the person saying the brocha obligated in his own brocha? How can it be a good brocha for the one reciting it and a ברכה לבטלה for the one being יוצא through שומע כעונה?
There are different ways amongst the meforshim how שומע כעונה works. Am I being יוצא with the דיבור of the one saying it, or, is it considered that through my listening it is as if I am saying it myself and being יוצא with my own דיבור? From this שער הציון it seems that through שומע כעונה it is as if the one listening is also saying the words and being יוצא with his own דיבור. Therefore if he is not obligated in the brocha it will be a ברכה לבטלה. He must therefore have in mind that he only wants to be יוצא with the brocha if he is still מחוייב in the mitzvah of sefiras haomer.
It is interesting to note that later on in ס”ק ל”ז the Mishna Berura does not say that you need to have this מחשבה of only being יוצא if you are still מחוייב. The מחבר (סעיף ח’) says that if one forgot to count one of the days, he should count the rest without a brocha. The reason is that there is a machlokes haposkim if every day of sefiras haomer is one mitzvah, or every day is its own mitzvah. Since according to some poskim if he misses one day he is no longer obligated to count, he should continue to count without a brocha. The Mishnah Berura says that if possible he should hear the brocha from someone else and answer amen with כוונה לצאת. The Mishna Berura does not say that you should think to yourself that you only want to be יוצא if you are still obligated in the mitzvah. From here it would seem that שומע כעונה is not that it is considered that I myself am saying the words and being יוצא with my own דיבור. Rather I am being יוצא with his דיבור, and since he is obligated to make the brocha it is not considered a ברכה לבטלה by answering amen if I am not obligated.
Prepared by R’ Avrohom Yehoshua Ziskind
 Sources:משנה ברורה,ביאור הלכה,שער הציון,דרשו,פסקי תשובות,ירח למועדים