The Oracle is one of the wisest characters in The Matrix, and although she doesn't directly tell Neo that he is "The One," she secretly gives him everything he needs to know. Although, by the end of the Matrix series, Neo is basically all-powerful, he is still trying to learn his potential when he makes his first visit to The Oracle. Because of this, she doesn't explicitly let Neo know what he is capable of, but one prop in The Matrix secretly tells him that he already is The One.
The Matrix follows Neo as he realizes that he is living in a simulation known as the Matrix, and he decides to break out and destroy the machines that are running it. During his journey, Neo encounters an altruistic program in the Matrix known as The Oracle, a seemingly all-knowing figure who can make incredibly accurate predictions about the future. Neo seeks her out in order to find out if he is The One, leading to one of The Matrix's most important scenes.
In the early parts of Neo's conversation with The Oracle, she begins talking about a woman who likes him, clearly referring to Trinity. She then analogizes being The One to being in love and, in doing so, points out a plaque she has hanging inscribed with the Latin phrase "temet nosce." The phrase actually means "know thyself," something she explains before switching back to talking about The One. After examining Neo, The Oracle tells Neo that he is not The One but says that "You got the gift, but it looks like you're waiting for something... Your next life, maybe." While this complex series of statements and analogies may be confusing at first, by the end of the film, it is clear that the phrase "temet nosce" means that Neo is The One all along.
By saying that Neo isn't The One but could be in another life, The Oracle alludes to events at the end of The Matrix. While Neo is fighting inside the Matrix, he gets injured and begins to die in the real world. While watching over his real body, Trinity kisses Neo, bringing him back to life and allowing him to fully emerge as The One. Neo has to die and come back to life at the end of The Matrix to realize his full potential, fulfilling the prophecy of The Oracle. Since it is still the same Neo, though, The One was inside him all along; he just had to know himself to bring it out. Although love is the analogy that The Oracle uses in order to explain being The One, Neo and Trinity's love is what literally allowed Neo to know himself and become The One.
Knowing one's true self is among The Matrix's many themes, but it is one of the most significant. The Oracle's "temet nosce" sign is key to Neo's journey, subtlety letting him know that he is really The One but must recognize it within himself. By explaining the significance of "temet nosce," The Oracle gives Neo the motivation he needs to recognize his potential, his love for Trinity, and his identity as The One, making him the hero of The Matrix.