Wikipedia says:

In Ophelia's first speaking appearance in the play,^[2] we see her with her brother, Laertes, who is leaving for France. Laertes lectures Ophelia against getting involved with Hamlet. He warns her that Hamlet does not have his free will as he is the heir of Denmark so does not have freedom to marry whomever he wants. Ophelia's father, Polonius, enters while Laertes is leaving, and Polonius also admonishes Ophelia against Hamlet, because he fears Hamlet is not earnest about her. Polonius concludes by forbidding Ophelia to have any further communication with Hamlet. She agrees to obey her father and to avoid Hamlet entirely.

In Ophelia's next appearance,^[3] she tells Polonius that Hamlet rushed into her room with his clothing askew, and with a 'hellish' expression on his face, and only stared at her, without speaking to her. Based on what Ophelia tells him, about Hamlet acting in such a "mad" way, Polonius concludes that he was wrong to forbid Ophelia to see Hamlet, and that Hamlet must be mad because of lovesickness for Ophelia. Polonius immediately decides to go to Claudius (the new King of Denmark, and also Hamlet's uncle and stepfather) about the situation. We later^[4] see Polonius suggest to Claudius that they can hide behind an arras to overhear Hamlet speaking to Ophelia, when Hamlet thinks the conversation is private. Since Polonius is now sure Hamlet is lovesick for Ophelia, he thinks Hamlet will express love for Ophelia. Claudius agrees to try the eavesdropping plan later. The plan leads to what is commonly called the 'Nunnery Scene'. ^[5]

In the 'Nunnery Scene' Polonius instructs Ophelia to stand in the lobby of the castle, while he and Claudius hide behind. Hamlet enters the room, in a different world from the others, and recites his "To be, or not to be" soliloquy. Hamlet approaches Ophelia and talks to her. He famously tells her "get thee to a nunnery." Hamlet becomes angry, realizes he's gone too far, and says "I say there will be no more marriage", and exits. Ophelia is left bewildered, scared, and heartbroken, sure that Hamlet is crazy. After Hamlet storms out, Ophelia makes her "O, what a noble mind is here o'erthrown" soliloquy.

The next time Ophelia appears is at the 'Mousetrap Play'^[6] which Hamlet has arranged to try to prove that Claudius killed King Hamlet. Hamlet sits with Ophelia and makes sexually suggestive remarks, also saying that woman's love is brief.

Later that night, after the play, Hamlet kills Polonius ^[7] - thinking Polonius is Claudius ("Is it the King?")- during a private meeting between Hamlet and his mother, Queen Gertrude. At Ophelia's next appearance,^[8] after her father's death, she has gone mad, due to what the other characters interpret as grief over her father. She talks in riddles and rhymes, sings some "mad" and bawdy songs about death and a maiden losing her virginity. After bidding everyone a "good night", she exits.

The final time Ophelia in the play is after Laertes comes to the castle to challenge Claudius over the death of his father, Polonius. Ophelia sings more songs and hands out flowers, citing their symbolic meanings although interpretations of the meanings differ. Then she blesses everyone and exits for the last time.

In Act 4 Scene 7, Queen Gertrude, in a famous monologue (There is a willow grows aslant the brook), reports that Ophelia had climbed into a willow tree, and then a branch broke and dropped Ophelia into the brook, where she drowned. Gertrude says that Ophelia appeared "incapable of her own distress" like a mad person would be. Gertrude's announcement of Ophelia's death is one of the most poetic death announcements in literature.^[9]

We later see a sexton at the graveyard insisting Ophelia must have killed herself,^[10] however, although the sexton attempts to argue the point logically and legally, he never says how he would know it as a fact. The cleric who presides at Ophelia's funeral later asserts that she should have been buried in unsanctified ground as a suicide, but he doesn't say how he knows facts about it, either. Laertes is outraged by what the cleric says, and replies that Ophelia will be an angel in heaven when the cleric "liest howling" (with the fiends in hell). The remarks by the sexton and the cleric have naturally led to a great deal of discussion of whether Ophelia committed suicide. Between Gertrude's report of an accident, and the later talk of suicide, the suicide issue is left unclear in the play, so that even after four centuries since the play was written, the issue is still a topic of debate.

At Ophelia's funeral, Queen Gertrude sprinkles flowers on Ophelia's grave ("sweets to the sweet,") and says she wished Ophelia could have been Hamlet's wife. Laertes then jumps into Ophelia's grave excavation, asking for the burial to wait until he has held her in his arms one last time, and proclaims how much he loved her. Hamlet, nearby, then challenges Laertes, and claims that he loved Ophelia more than "forty thousand" brothers could. After her funeral scene, there is no further mention of Ophelia.

Sparknotes writes: