Does a minyan say tachanun if there was a siyyum or if there is going to be after aleinu? Or, is it preferable to have a Siyum on Shabbat (just for example) to avoid this safeik?
Is Tachanun recited at a Siyum of all the Masechtot of Sha"S Bavli?
Rav Ovadia Yosef (Yabia Omer OC 4:13) discusses this topic and concludes in favor of not saying tachanun at a siyum in order to emphasize the aspect of happiness when learning Torah and thereby reward those who do learn Torah / entice others to learn Torah too.
I personally made a siyum on Shas right after mincha, and R' Herschel Schachter paskened that we shouldn't say tachanun during mincha.