Is expiation obligatory for every oath sworn by Allah, and when does it become obligatory? And does the statement: "By Allah, if Allah wills," confirm the oath?

One should not swear excessively, as it indicates a lack of reverence for Allah. Allah Almighty has indeed condemned those who swear frequently. As for the expiation for an oath (kaffarat al-yameen), it becomes obligatory for a binding oath (al-yameen al-mun'aqidah), which is an oath concerning a future action made with the intention of emphasizing it. The expiation offers a choice between freeing a slave, or feeding ten poor people, or clothing them. Whoever cannot find these options must fast for three days. If a person swears an oath concerning another, the expiation falls upon the one who swore the oath. There is no expiation for a vain oath (yameen al-laghw), which is an unintentional oath or one where the swearer believes it to be true but it turns out to be otherwise. Fasting is not sufficient for expiation except when one is unable to fulfill the other options. Saying "Insha'Allah" (if Allah wills) in an oath is considered an exception that prevents the breaking of the oath and its expiation.