Arguably, the best part of the Harry Potter series is it's rich and diverse cast of characters. Although everyone can agree that the main antagonist, Voldemort, was evil, there is far more to his personality than what you see at the surface level.
Openness: moderate-high
Voldemort scores high in this area in the sense that he is extremely smart and intellectual. He was hungry for knowledge and power, and not afraid to go to the darkest corners of the world to find what he wanted. However, he has a deep-seated hatred for muggles, showed him to be close-minded.
Conscientiousness: very high
Voldemort had great self-discipline and went to great lengths to plan every detail of his schemes. Although he had no regard for the rules, while a student at Hogwarts he showed he was able to achieve great academic success and to follow the rules carefully, even rising to the role of prefect.
Extroversion: low
In public, Voldemort may have seemed extroverted. However, most of his plans were known only to himself. Because he hated others, he did not enjoy their company, and spent much of his time scheming alone. His most prized possessions were his horcruxes, which in fact were extensions of his own self.
Agreeableness: very low
When it suited his way, Voldemort was able to act agreeable in order to manipulate others. However, he was actually a very cold and calculating person. He had no regard for others' feelings, and treated them as disposable objects. In this sense, Voldemort was almost on the level of a psychopath.
Neuroticism: moderate-high
Voldemort was usually able to keep his emotions well hidden from others. However, he was prone to bursts of rage when things did not go his way. He also often let his true fear slip, especially during times when Harry Potter got the upper hand.
This is a pretty good assessment of Voldemort using the OCEAN personality traits. The only thing I would like to add may be that in the extroversion paragraph, I agree that he is low in extraversion, but most of his plans were not kept entirely to himself because he had his Death Eaters and even Draco Malfoy carry out most of his deeds for him, so they were all well-informed of most of his plans. Other than that, I liked it and it was an interesting read!