Is the Mitzvah of Sefiras Haomer Biblical or Rabbinical?
Temple times: It is a positive command of the Torah for every single Jew to count seven weeks’ worth of days beginning from when the Omer was offered.
In the Diaspora: The Mitzvah of counting the Omer Biblically applies in both Eretz Yisrael and the Diaspora.
During exile is the Mitzvah of Biblical or Rabbinical status? It is disputed as to whether the obligation of counting the Omer during the times of exile, when we no longer have a Temple and no longer bring the Omer offering, is of Biblical requirement or is a Rabbinical injunction. Practically, the main Halachic opinion follows like those who hold that the Mitzvah today is of Rabbinical status. Nevertheless, despite its Rabbinical nature, the institution of the Sages follow the same laws as the Biblical command and there is thus no practical Halachic differences between them, other than those cases to be mentioned. [These differences include a) If during the middle days one did not count both the days and weeks but rather only the days or only two weeks, in which case we rule that Bedieved one fulfills his obligation as Safek Derabanon Lehakel. b) That from the letter of the law it is permitted for one to count by Bein Hashmashos, as Safek Derabanon Lehakel, although initially one should be stringent to count after nightfall in order to suspect for the opinion who says that the counting is Biblical even today.]
If one intended to fulfill the Biblical command of Sefirah upon counting, is he Yotzei?
Seemingly ,one is not Yotzei as he has transgressed Baal Tosif, nevertheless he may continue to count with blessing. Accordingly, some discourage the recital of the Nussach of Lisheim Yichud for Sefiras Haomer being that it can lead one to intend to do the mitzvah on a Biblical level which is forbidden during this time.
 Admur 489:1
 Admur ibid; Michaber 489:1; Menachos 65b; This is coming to exclude the idea that only the Beis Din is obligated in the counting. It however is not coming to include women within the obligation, as women are explicitly exempt from this Mitzvah as explained in 489:2.
 Admur 489:2; Rambam Temidim Umusafim 7:24; Ravayah 526
 Admur 489:2
 The dispute: [According to some opinions] this [Biblical] Mitzvah of counting the Omer [Biblically] applies even [during exile] when we do not have a Temple. [1st opinion in Admur ibid; Rambam Temidim Umusafim 7:24; Ravayah 526; Chinuch 306] However, other opinions rule that today in exile, when we no longer have a Temple or bring the Omer offering, the Mitzvah of Sefira is no longer at all a Biblical command and is rather a Rabbinical obligation that was instituted in memory of the Temple. [2nd opinion in Admur ibid; Tosafus Menachos 66a; Rosh Pesachim 10:40; Ran Pesachim 28a; See Beis Halevi 1:39] Other opinions rule it is no longer Biblical because we do not have the Temple and cannot bring the Shtei Halechem on Shavuos, [and not because of the Omer offering]. [Likkutei Sichos 38:10 [printed in Toras Menachem 3:4-12] and Toras Menachem 5751 3:95; Radbaz 1327; Abudarham Seder Tefilos Shaar 3; Maharil Seder Birchas Haomer; Beis Yitzchak Y.D. 2:19; Sefer Hamachria 18; Nimukei Orach Chaim in end of Sefer; Darkei Chaim Veshalom 625; Minchas Elazar 5:1; Piskeiy Teshuvoas 489:10] The Ran ibid explains an alternative reason for why the Sages established the Mitzvah of Sefira today in exile is because even originally the Mitzvah of Sefira was not relevant to the Omer or Temple, as the Jewish people first began counting the Omer in anticipation of receiving the Torah, and hence today too we count the Omer in anticipation for Shavuos. Nevertheless, this is merely a Midrash, as the main reason for the institution was for commemoration of the Temple. [Ran ibid]
 Admur ibid; Beis Yosef 489; implication of Michaber 489:6; M”B 489:14; Kaf Hachaim 489:4
 See 489:6; See Halacha 7B
 See 489:12-13; See Halacha 3C
 See Orchos Chaim 489:4 in name of Meoreiy Or
 See Likkutei Sichos 1:271
 See Yesod Veshoresh Havoda 9:8; Mishmeres Shalom 36:2; Orchos Chaim 489:4; Shaar Yissachar Nissan Omer Hatenufa 17; Darkei Chaim Vehsalom 625; Dvar Moshe 29; Kaf Hachaim 489:7; Piskeiy Teshuvos 489 footnotes 46-49