Post History

Many Rishonim give reasons why a blessing is not said by women monthly when counting clean days (eg. she may bleed again and break the count ([Tosafot Menachot 65b](https://hebrewbooks.org/shas.aspx?mesechta=30&daf=65b)) or because nowadays we only keep clean days because of doubts how to correctly count Nidda vs Zava ([Raavyah 526](https://hebrewbooks.org/pdfpager.aspx?req=14535&st=&pgnum=184))) which only apply to the blessing but seem to assume the count itself is/was still actually a mitzva. [Sefer Aguda](https://hebrewbooks.org/pdfpager.aspx?req=9770&st=&pgnum=444) even quotes [Ri](https://en.wikipedia.org/wiki/Isaac_ben_Samuel) explicitly that women should count orally. Those that don't include it in their list of 613 commandments may have included it in the general mitzva of זב/זבה or as a detail in the commandment to immerse in the mikva.

Radvaz ([Responsum #1102](https://www.sefaria.org/Teshuvot_HaRadbaz_Volume_4.1102)) says in this case, since one could choose to remain impure, the verse can't be referring to an obligation and we must thus understand it refers to general calculation without all the details of counting the Omer.

Another option is that וספרתם לכם by Omer doesn't alone imply oral counting, and it's only from the repetition תספרו חמשים יום and/or שבעה שבועות תספר לך that we derive the additional oral obligation (see for example [Netziv](https://www.hebrewbooks.org/pdfpager.aspx?req=14022&st=&pgnum=191)). Accordingly, "counting" clean days remains a calculation alone.