Zeffirelli adds the Oedipus Complex to the film and one reason I think Zeffirelli adds this because he wanted more of a relationship for Hamlet other than Ophelia.
Zeffirelli knew Ophelia dies in the play as told by Gertrude in Act 4 Scene 7 so I believe that's why in the film we don't see much of relationship between Hamlet and Ophelia there rather distance and only on screen together a couple times with little interaction. Knowing Ophelia dies rather early on in the film Zeffirelli wants Hamlet to make some sort of connection romantically because in film romance enhances drama so he picks Hamlet’s mother. The Oedipus Complex in Hamlet has been debated by scholars for years and here Zeffirelli on his own take of Hamlet just goes with it. In the perspective of a writer and director Hamlet's relationship with Gertrude does add more to their deaths at the end of the film as they die near each other and Ophelia dies completely on her
own. Zeffirelli adds the Oedipus Complex to the film and another reason for that is because it adds more tension between Claudius and Hamlet. Without the Oedipus Complex there's still tension between Claudius and Hamlet but added Gertrude into the mix adds a triangle of problems. Hamlet is now angry at Claudius for killing his father, marrying his mother so early after his brother's death and marrying his mother because he would rather do so. Adding more tension between Claudius and Hamlet makes Claudius’s death more enjoyable to Hamlet and to the audience. Zeffirelli adds the Oedipus Complex to the film and the last reason is because it adds more to Gertrude's character. In the text, Gertrude’s character doesn’t play a very large role but in the film, you could say she plays the second largest role in Hamlet. Zeffirelli adding the Oedipus Complex makes Hamlet and Gertrude more detailed as characters, in the text Gertrude and Hamlet's relationship is weak they don’t talk much and the times they do talk it’s not the real Hamlet it’s the crazy “acting” Hamlet or when they do indeed talk it's usually talking about someone else or their conversation doesn’t really show who they are as characters. But in Zeffirelli's Hamlet Gertrude is given a larger role and also has as much screen time as Claudius and Ophelia, The Oedipus Complex help give Gertrude more than Shakespeare intended but I think it was a good change. In conclusion Zeffirelli's changes to the original text by adding the Oedipus Complex and he does so to add more romantic drama to Hamlet, to add more to for the Hamlets hate of Claudius, and lastly give Gertrude a strong effect and presents in the film. Hamlets Zeffirelli is far from perfect but it was a good film it told Shakespeare's play on a different way and added in the Oedipus Complex was a bold move, the film is subjective either you like it or you don’t it’s all up to the viewer.