While Arjun and Ovi were in Canada they got engaged. Ovi always loved Arjun since she was a child, whereas Arjun did not.
Arjun came to India for business matters and bought a company where Purvi worked. While working/fighting together they fell in love.
Arjun wanted to marry Purvi and everyone sooner or later agreed with him, but Ovi wouldn't let go.
Ovi bribed Purvi to let her marry him and in return she will love Archana. Purvi agreed.
Ovi always suspected Arjun and his feelings ever since they got married. She would pick fights with Purvi.
Purvi left everyone and went to Kolkata to help their marriage. Six months passed and she married Onir, but her child is Arjun's (this point has not been shown in the show yet).
I hope I helped! 😊