Melacha prior to counting:
It is accustomed for both men and women to avoid doing work/Melacha during the entire period of Sefiras HaOmer, from the time of sunset until after they have counted the Omer. (Possibly those women which are not accustomed to count the Omer must refrain from work throughout the entire night) [For this reason women should continue to count the Omer nightly even if they can no longer say a blessing due to missing a day, as otherwise they cannot do Melacha until morning.] [Despite the above ruling, many are not accustomed to refrain from Melacha during this period of time and there are Poskim which have defended their actions.]
What form of work is to be avoided?
 493/9; Michaber 493/4
 In the Michaber [493/4] men are omitted, and the custom is only brought regarding women. Nevertheless the Kneses Hagedola adds that the custom applies likewise to men, and so rules Admur. Some explain the reason the Michaber omitted the custom for men is because men are in any event forbidden from doing Melacha from sunset until they count, as is the general law regarding any Mitzvah which arrives, such as Bedikas Chameitz, and prayer. [Olas Reiyah 52]
 The reason: This is hinted to in the verse “Sheva Shabasos” and Shabasos has the same terminology as the word rest, hence implying that from the time of the counting of Sefira, which is from sunset, until one counts, one is to rest from doing Melacha. [Admur ibid; this is the 2nd reason mentioned in the Tur and is based on the ruling of Taz]
Other reasons mentioned: The students of Rebbe Akiva were buried during the period of Sefira after sunset, and it is accustomed to avoid doing Melacha while a Levaya/funeral is in procession. Based on this reason one is to refrain from Melacha the entire night, even after counting. [1st reason in Tur and M”B 493/19] Another reason: The period between Pesach and Shavuos is similar to Chol Hamoed, as Shavuos is similar to Shemini Atzeres of Pesach, and thus in commemoration of this the Sages enacted to avoid Melacha after sunset. [Yaavetz in Mor Uketzia]
 Olas Reiyah 52
 Chok Yaakov 493/12 writes he has not seen people careful in the above custom; Beis David [271 brought in Kaf Hachaim 43] rules there is no obligation to follow this custom, and those which were not accustomed to do so are not to be protested.
 Kaf Hachaim 493/45
 Piskeiy Teshuvos 493/16