Is an oath or vow to say maariv daily for a man a valid oath? Or is it not a valid oath since maariv is considered obligatory at present for men and we are under oath from the Torah to pray?
Shalom, Thank you for your interesting question. I assume you are asking based on the fact that an oath taken to perform a Torah obligation is not a binding oath. But an oath taken to perform a Rabbinic obligation does take effect, and the oath applies above and beyond the Rabbinic mitzvah (see Shulchan Aruch, Yoreh Da'eh, 239,6). The mitzvah to say Shema in the evening service is a Torah obligation. However, I assume you are asking about the rest of the evening service, especially the Amidah. The Talmud (Berachot 27b) debates this point, and we rule that maariv is "reshut" - optional. Even so, we have taken it upon ourselves as an obligation (see Rambam, Tefillah, 1,6). This is why all men pray the evening service every night. This obligation, though, is not a Torah one, and therefore an oath about it would indeed take effect. (I did not find any sources that address this question specifically, and base my answer on the discussion of the level of obligation of maariv). It goes without saying that we exercise extreme caution to avoid making oaths and vows. And also that one must pray the evening service despite any discussion of its level of obligation. Blessings.