The Mishna Berura (498:9) writes that when one counts the omer after the sixth day, when reference is made to both the number of days and the number of weeks, the word “shavua” must be pronounced “shAvua,” with a kamatz vowel, and not “shEvua,” with a sheva vowel. (Presumably, this would apply also to the plural form “shavuot,” which must be pronounced “shAvuot,” and not “shEvuot.”) The reason, as explained by Rav Efrayim Greenblatt in his Rivevot Efrayim (5:344), is that the word “shevua” means “oath,” and not “week,” and is thus incorrect in the context in the sefirat ha-omer.
In his discussion of this topic, Rav Greenblatt writes that in his view, one who mistakenly says “shevuot” has not fulfilled his obligation, and must repeat the counting, since he mispronounced the vitally important word “shavuot.” However, he cites other authorities with whom he had corresponded about this subject who disagreed, and ruled that the mitzva is fulfilled even if one mistakenly recited “shevuot.” One correspondent, Rav Shushan Mazuz, noted the fact that, as the Mishna Berura (489:7) mentions, some authorities ruled that one who counted only the days (for example, “Today is the eighth day of the omer”) without mentioning the weeks (meaning, he did not add, “which is one week and one day”), has nevertheless fulfilled his obligation. These poskim include the Magen Avraham, the Chid’a, the Chok Yaakov, the Elya Rabba, and others. In light of the different views on the subject, the Mishna Berura ruled that one who omitted mention of the weeks should repeat the counting, but without a berakha, in order to satisfy all opinions. In the case of one who mistakenly recited “shevuot,” Rav Mazuz writes, we can factor in the lenient view noted by the Mishna Berura, as well as the fact that it is uncertain whether the mispronunciation of “shavuot” undermines the counting of weeks. Therefore, although it would be preferable for the individual to repeat the counting without a berakha, he has fulfilled his obligation.
Another consideration, which Rav Greenblatt cites from Rav Yosef Bar Shalom, is that even one who mistakenly recited “shevuot” clearly meant to say “weeks,” and not “oaths,” and thus he has fulfilled the mitzva despite the mispronunciation. Rav Bar Shalom added that according to Rabbenu Yerucham, the requirement to mention the weeks in the counting to begin with applies only on the level Rabbinic enactment (even if sefirat ha-omer nowadays applies on the level of Torah obligation), so there is certainly room for leniency in this regard.