When looking up information about a favorite anime or manga character, it isn't uncommon to stumble upon their blood type. While this may seem odd to some, it's actually part of a popular form of fortune-telling. In 1926, Rin Hirano and Tomita Yashima published a paper explaining their belief that a person's blood type is a good indicator of their personality. While this paper has been thoroughly discredited, the Blood Type Personality Theory quickly caught on in many countries, with some using blood types like Americans use the Zodiac signs.
While creating Sailor Moon, Naoko Takeuchi spent a lot of time thinking about each character's personality and lore. Part of this was ensuring each character had an officially-listed blood type and zodiac sign that fit their personality and power set. Now, using this information, Sailor Moon fans can see Usagi herself in a whole new light.
Usagi is listed as having O-Type blood. This is actually fitting from a medical perspective, as people with O-Type blood (specifically O negative) are referred to as universal donors. In other words, their blood can be transfused into people with other blood types without risk of an incompatibility reaction. The description fits Usagi's role as, like her blood, she can save anyone and everyone in distress with her Sailor Moon powers.
People with O-Type blood are said to be romantics and idealists, which suits Usagi's personality perfectly. This is shown by her regular romantic daydreams about Mamoru, or how flustered she gets while daydreaming about Sailor Uranus when they're handcuffed together. Her romanticism also comes through in her view of others, as Usagi often tries to see the best in people even when they're doing horrible things. These qualities are best shown with Black Lady, as Usagi never gives up on her goal of returning Chibiusa to normal and refuses to believe believe her time-displaced daughter is gone.
O-Types are also said to be good at overcoming adversity, which does fit Usagi, especially as time goes on in Sailor Moon. But even at the story's beginning, Usagi is shown overcoming her fears to protect those she cares about. In the first episode, she fights a terrifying creature to rescue her friend Naru. This trend continues into the fight with Queen Beryl. Despite her friends getting killed and the odds not being in her favor, Usagi doesn't give up hope and continues to fight.
Most blood type theories say that O-Types are good leaders, which describes Usagi in some ways. As Sailor Moon progresses, Usagi grows into her role as the leader of the sailor scouts. By the end, she is willing to lead the charge against her foes, as seen when she finally faces down Chaos. At the same time, she rarely is a leader in the traditional sense, and the Sailor Soldier team has a pretty flat hierarchy. Usagi does become Neo-Queen Serenity in the future, which is very clearly a role of leadership, so it could be argued this fits her personality arc in the long-term -- even if viewers don't get to see much of it.
Despite all this, Usagi deviates from a traditional O-Type in one massive way: O-Types are often described as easygoing and, while devoted, they traditionally don't get hung up on the small stuff. This is the opposite of Usagi, who is easily flustered and stressed, especially when it comes to schoolwork. In fact, her crybaby reputation shows that everyone around her knows she isn't easygoing at all. While Usagi's scatterbrained nature does fit with this trait, episodes like "Aim to Be a Princess? Usagi's Weird Training" show how she doesn't like forgetting schoolwork and is slightly frustrated by her own inability to focus, making her less easygoing and more easily distracted.
Blood Type Personality Theory is a fascinating way to analyze Sailor Moon, as Naoko Takeuchi clearly spent a lot of time working out each character's personality. While Usagi doesn't totally fit the traditional O-Type archetype, especially later in the story, her type does give us a glimpse into Takeuchi's thought process as she was creating the character. It also suggests the creator had a lot planned for Sailor Moon -- even when she had just begun writing the epic tale.