O Kadhal Kanmani (transl. Oh love, apple of my eye), also known as OK Kanmani, is a 2015 Indian Tamil-language romantic drama film written, directed and produced by Mani Ratnam. The film stars Dulquer Salmaan and Nithya Menen, portraying a young couple in a live-in relationship in Mumbai. Prakash Raj and Leela Samson play supporting roles as an older couple in the film, portraying the younger couple's landlords. The film was said to be a "reflection of the modern mindset of urban India", dealing with issues such as marriage and traditional values. The film featured music composed by A. R. Rahman, cinematography by P. C. Sreeram and editing by Sreekar Prasad. The title of this film is borrowed from the song 'Pottu Vaitha Kaadhal Thittam' from Kamal Haasan's 1992 film Singaaravelan.

Prior to the film's release, the soundtrack was well received and the promotional activity of the film on social media also garnered praise. O Kadhal Kanmani was released on 17 April 2015 and won positive reviews from critics, praising the lead pair's performances and the quality of the film's technical aspects. The film became a financial blockbuster at the box office, performing extremely well in Indian multiplexes and overseas. The film was later remade into Hindi as Ok Jaanu in 2017. O Kadhal Kanmani has gained a cult status over the years.