An oath is a promise, a pledge, a kasam or a vaada. A promise is usually sincere, unless it is a politician making it.
Societies tend to respect promises; to appear more convincing we often say “God promise”. In Hindi films, a “maa ki kasam” has even more wazan.
The two biggest recipients of promises are your lover and your God. In a fit of despair, when a dear colleague and his wife were both diagnosed with cancer on the same week in 2000, I entered a temple and dropped some money in a hundi for their protection. The man lives, the wife died, and God thus conveyed that he too plays dice.
Hindi film songs abound with promises, both met and broken. Here’s Tariq (Rafi) asking his love what happened to her promise  in Hum Kisise Kum Nahin (1977), and here’s another lovely pledge  that will please everyone from Feroz Khan (Mukesh) in Safar (1970). Another song I like, chiefly because of Moushumi (Lata-Kishore), is this vow to be together  from Subse Bada Rupaiya (1976); if the song sounds R D Burman-ish it’s because the composers are his chamchas Basu Manohari. An asli RD-Kishore-Lata tune that has aged well (as has Sharmila) is another promise to always stay together  from Aa Gale Lag Jaa (1973). This compilation needs at least one lilting tune from old man Burman and that’s provided by Sharmila (Lata)’s vow  in Talash (1969).
I could go on. That pledge never to forego a friendship  from RD-Kishore-Manna in Sholay (1975), and, for completeness, let me mention an ordinary KA-Rafi-Lata song about always being together  from Haath Ki Safai (1974) that gained seriously in popularity. Then there’s the breezy, but convoluted, O P Nayyar-Rafi-Asha duet from Tumsa Nahin Dekha (1957) in which Ameeta swears to Shammi Kapoor that he’ll regret missing her company  in Tumsa Nahin Dekha (1957), and the intensely melodious duet on the bicycle  by SDB-Kishore-Lata from Tere Mere Sapne (1971). And I shouldn’t forget Kaka gyrating  in that Dushmun (1972) qawaali with LP-Kishore.
Ok, we’re now down to my top two. And after some reflection, and more trepidation since it’s such a lovely song, I have decided to have jo vada kiya woh nibhaana padega  from Taj Mahal (1963) at second place. My top pick is the brutal and haunting Manna song from Upkar (1967). Pran plays the good guy for the first time in this Manoj Kumar film, KA have a winning tune, but Indivar makes this good song truly great. https://www.youtube.com/watch?v=vyrOdkr2myw
Kasme vaade pyaar wafa sab baate hain baaton ka kya?
Koi kisi ka nahin, ye jhuthe naate hain, naaton ka kya?
[Vows, promises, love, faith … all these are just hollow words
No one lives for anyone else, all relationships are empty and meaningless]
Hoga masiha saamne tere, fir bhi na tu bach payega
Tera apna khoon hi akhir, tujh ko aag lagayega
[The Messiah may be right in front of you, but even He can’t save you
Your own dear one will set your destiny on fire]
Sukh mein tere saath chalenge, dukh mein sab mukh modenge
Duniyawale tere bankar, tera hi dil todenge
Dete hain bhagwan ko dhokha insaan ko kya chhodenge
[They walk alongside in happy days, but desert you as soon as the bad times hit you
They’ll pretend to be one of you, and then break your heart
When they betray the Lord Himself, imagine what they’ll do to mere mortals]
 Tariq-Kaajal Kiran-Rishi Kapoor + Rafi + RDB-Majrooh. Hum Kisise Kum Naheen (1977)
 Feroz Khan-Sharmila Tagore + Mukesh + KA-Indivar. Safar (1970)
 Moushumi Chatterjee-Vinod Mehra + Lata-Kishore + Basu Manohari-Majrooh. Sabse Bada Rupaiya (1976)
 Shashi Kapoor-Sharmila Tagore + Kishore-Lata + RDB-Anand Bakshi. Aa Gale Lag Jaa (1973)
 Sharmila Tagore-Rajendra Kumar + Lata + SDB-Majrooh. Talash (1969)
 Dharmendra-Amitabh Bachchan + Kishore-Manna + RDB-Anand Bakshi. Sholay (1975)
 Vinod Khanna-Simi + Rafi-Lata + KA-Gulshan Bawra . Haath Ki Safai (1974)
 Shammi Kapoor-Ameeta + Rafi-Asha + O P Nayyar-Majrooh. Tumsa Nahin Dekha (1957)
 Dev Anand-Mumtaz + Kishore-Lata + SDB-Neeraj. Tere Mere Sapne (1971)
 Rajesh Khanna (Kaka)-Bindu + Kishore + LP-Anand Bakshi. Dushmun (1972)
 Pradeep Kumar-Bina Rai + Rafi-Lata + Roshan-Sahir. Taj Mahal (1963)