Om Prakash Makhija (Shah Rukh Khan) is a junior artist in the 1970s Hindi film industry, in love with film star Shanti Priya (Deepika Padukone). One evening, Om views the premiere of Shanti's film Dream Girl, posing as Manoj Kumar. Later that night, Om becomes drunk and describes his vision of himself as a famous, wealthy film star winning the Filmfare Award for Best Actor to his friend Pappu (Shreyas Talpade) and some local children. At the shooting of a later film, Om rescues Shanti from a fire scene where the fire has escaped control, and they become friends. One day, after following Shanti, he overhears her and producer Mukesh Mehra's (Arjun Rampal) conversation, that Mukesh and Shanti have married in secret and Shanti is pregnant and expects a traditional wedding. Mukesh pretends to be happy with the news, and invites Shanti at the set of their soon-to-be-made film Om Shanti Om. Mukesh promises to cancel the film, reveal their marriage to the public, and have a grand wedding on the set; but instead traps Shanti on the set and sets it on fire to prevent a financial loss and to protect his career. Shanti tearfully begs Mukesh to release her but he leaves anyway.
Om, who happens to be at the same location that night, attempts to rescue Shanti, but is attacked by Mukesh's guards. After the guards leave, he again attempts to rescue Shanti and enters the set. He finds Shanti but he is thrown from the building by an explosion. The building explodes after he is thrown from it and Shanti appears to have died.
Outside, Om is hit by a car owned by Rajesh Kapoor (Javed Sheikh), a well-known actor taking his wife Lovely to the hospital to give birth. At the hospital, Om remembers his moments with Shanti as he dies due to these injuries while Lovely gives birth to a son, also named Om.Om reincarnates as Om Kapoor and sets out to avenge Shanti's death. He is nicknamed "O.K.", and lives the luxurious life imagined by Om Prakash, but experiences pyrophobia and subconsciously inherits Om Prakash's memories. In acceptance of an award, O.K. unexpectedly delivers Om Prakash's drunken speech to the assembly. Pappu hears this on television. At a celebration of his award, O.K. is introduced by his father to Mukesh Mehra, and remembers the events that happened in his past life thereupon completely recovering Om Prakash's memories. He later reunites with Om's mother Bela (Kirron Kher) and Pappu and conspires to avenge Shanti's death by making Mukesh confess his crime.
To that end, O.K. convinces Mukesh to restart Om Shanti Om, while scheming to convince Mukesh that Shanti's spirit is haunting him. To impersonate Shanti, O.K. finds Sandhya, a.k.a. Sandy, a doppelgÃ¤nger of Shanti. Throughout the film's shooting, O.K. and his friends arrange incidents to remind Mukesh of his crime. During the music launch of the film, O.K. taunts Mukesh by revealing, through a song, the extent to which he knows the story of Shanti's death. But when Mukesh runs after Sandy thinking she is a ghost of Shanti, Sandy accidentally cuts her arm and bleeds. Seeing this, Mukesh realises that she is not Shanti's ghost. But when he is about to confront O.K, he is suddenly hit by the set's chandelier.
After Mukesh wakes up, O.K. confronts Mukesh but is shocked when Mukesh reveals that he knows who Sandy is. During the quarrel, Sandy shows up. O.K. tries to send Sandy back. Sandy reveals that Mukesh, after the fire ceased to burn, found that Shanti survived the fire, but he buried her alive below the chandelier. O.K. is confused about how Sandy knows this. Mukesh attempts to shoot Sandy, shocked by the extent to which she knows about the murder, but Mukesh and O.K. fight, which results in yet another fire. Just when O.K. is about to kill Mukesh, Sandy stops him, says that Mukesh will not die by his hand, and the chandelier drops on Mukesh, killing him instantly. Pappu and Sandy join O.K. O.K. is shocked when he sees the real Sandy and realises the person he assumed Sandy was actually Shanti's soul. The soul smiles warmly towards Om and tearfully bids him goodbye . The film ends with Shanti's soul leaving the scene and disappearing into the light.
Who hit Om with their car?
Rajesh Kapoor