Eating before counting:
One must refrain from eating even a small a meal a half hour prior to the time of the Sefirah, which is a half hour before Bein Hashmashos [i.e. sunset]. This applies even if one has already Davened Maariv but has not yet recited Sefiras HaOmer.
If one has someone to remind him: If one lives in an area in which the Shamash is accustomed to remind others to recite the Sefirah upon the time of Sefirah arriving, then one may begin to eat [within the half hour prior to sunset]. [The same allowance applies if one appoints a Shomer/guard to remind him to count Sefira when the time arrives. Likewise, if one has a set Minyan in which he Davens Maariv upon nightfall, then he may eat within the half hour prior to sunset. However once the time [of sunset] has arrived it is implied from the Poskim that this allowance no longer applies, and hence even if one has a Shomer he must refrain from eating until he counts. Some Poskim however rule that if one has a Shomer or a set Minyan then he may eat prior to counting even after nightfall.]
If one already began his meal: If one did not appoint someone to remind him to count and forgot or transgressed and already began his meal within the half hour before sunset, then he is not required to stop his meal and count the Sefirah [even after the time arrives]. If however one began his meal after the time of Sefira [i.e. after sunset] then he must stop and count the Sefirah in middle of his meal.
May one snack prior to counting Sefirah?
Definition of snack: The definition of a snack is up to 57.6 grams of bread [or Mezonos] and 57.6 grams worth of an [alcoholic] beverage. One may drink an unlimited amount of other liquids, such as water, tea, and coffee, and eat an unlimited amount of fruits.
If one Davens Maariv with a set Minyan every night may he eat a meal prior to counting Sefirah?
If one Davens with a set Minyan at a specific time every night then one may eat a meal up until sunset. Regarding eating a meal past sunset in such a case: See above that although from the Shulchan Aruch it is implied that one may not eat a meal past sunset even though he has a reminder, nevertheless some Poskim rule that if one has a set Minyan then he may eat prior to counting even after nightfall, and so is the widespread custom regarding Maariv throughout the year.
May women eat prior to counting Sefirah?
Those women that have not accepted the Mitzvah upon themselves as an obligation are not required to refrain from eating beforehand. However those women who have accepted this Mitzvah upon themselves as an obligation are to follow the same law as men.
 489/17; Rama 489/4; M”A 489/11
 Admur ibid; M”A 489/11; Birkeiy Yosef 232/5; Elya Raba 48922; Biur Hagr”a; M”B 489/23; Kaf Hachaim 489/64
Other opinions: The Rama 489/4 simply states “One must refrain from eating when the time of Sefira arrives” and does not mention “a half hour prior to this time”. The M”A ibid however questions the Rama ibid and concludes that one is to be stringent within the half hour before.
 Admur ibid; M”A 489/11; Kaf Hachaim 489/64
The reason: This is due to a decree that one may come to drag out his meal and hence refrain from counting the Omer altogether. [Admur ibid]
Other opinions: Some Poskim however rule that one is to be stringent only 30 minutes prior to nightfall, and not 30 minutes prior to sunset. [M”B 489/23] The Kaf Hachaim ibid concludes like Admur, as so is implied in the M”A ibid.
 This is common amongst those that are accustomed to Daven Maariv while it is still day and then read the three Parshiyos of the Shema prayer prior to going to sleep. Thus the novelty here is that although it is permitted for him to eat prior to reciting Kerias Shema Sheal Hamitah, as explained in Taz 235/3 and M”A 235/2 and 5, nevertheless during the days of Sefira he may not begin to eat starting from a half hour before sunset. [Admur ibid]
 Admur ibid; M”A ibid; M”B ibid
 The reason: As in such a case there is no suspicion that one may come to continue his meal past the time of counting Sefira, as explained in Rama 232/2. [Admur ibid; M”A 489/11]
 Kaf Hachaim 489/63
 Piskeiy Teshuvos 489/16; Regarding the Heter of “Minyan Kavua” see Michaber 89/6; Admur 89/7; Aruch Hashulchan 232/15; Kaf Hachaim 232/29; Igros Moshe 4/99; SSH”K 56 footnote 12;
 Admur ibid; M”A ibid
 Igros Moshe 4/99; Az Nidbaru 6/52
 The reason: As even regarding Shema of Maariv, which is Biblical according to all, a Shomer or set Minyan suffices to allow eating past dark, prior to praying. Thus how can Sefiras Haomer which is today Rabbinical be more severe than the Shema? Now, one cannot answer that one should simply say the Sefira before eating [and before Maariv] as it is important to say the Sefira together with a Minyan, after the Maariv prayer, as explained above. [ibid]
 Admur ibid in parentheses; Rama ibid regarding if began before the time of Sefira, however the Rama ibid does not hold of a transgression in beginning a meal within the half hour, however according to Admur ibid there is a transgression, and hence the novelty here is that despite the transgression, he may continue his meal.
 Admur ibid; Rama ibid
The reason: The reason for why one must stop in middle of his meal is because there are opinions which rule that the counting is a Biblical Mitzvah of which we rule in M”A 235/5 that one must stop his meal in order to fulfill a passing Biblical Mitzvah, such as Shema. Now, although the main ruling follows those that hold that today the Mitzvah of Sefira is Rabbinical, nevertheless regarding a matter that holds no difficulty at all, such as to stop a little and count the Sefira, certainly one should suspect for their opinion. [Admur ibid; Chok Yaakov 489/17]
 Mishna Berurah 431/6 regarding Bedikas Chametz; See also regarding Megillah: M”A 692/7; Elya Raba 692/11; M”B 692/14; Kaf Hachaim 692/36
 Peri Megadim 431 Ashel Avraham 4; Piskeiy Teshuvos 692/8 based on P”M 431 A”A 4
 Piskeiy Teshuvos 692/8 based on M”A 232; M”B 232/35
 M”B 692/14 in name of Derech Hachaim 3; Kaf Hachaim 692/36
 Mishna Berurah 431/6 regarding Bedikas Chametz; Regarding Megillah: Piskeiy Teshuvos 692/8 based on M”A 232; M”B 232/35
 See regarding Shofar: Ashel Avraham Butchach 689 and Mahadurah Tinyana 592