Oliver Queen was the Arrowverse's titular vigilante, but he didn't become a true hero until Arrow season 4. For the first few years of his time as the Hood and Star City's protector, Oliver Queen was more of an anti-hero; his first and only goal was simply to cross off the names on his hit-list with lethal force. And so, it would be a long road to becoming a true hero seeing as how Oliver wasn't even concerned with being one.
The training Oliver received over the years prepared him to be more of a killer than a savior. And between Lian Yu and the Bratva, it was a given that he would be harboring guilt, trauma, and darkness. This made him vulnerable to manipulation a few times by villains like Slade Wilson and Ra's al Ghul. But Arrow season 4 saw a tremendous turn in him, from the human weapon he was trained to be, as he started to understand that he needed light instead of darkness to beat Damien Darhk.
Oliver Queen's first few villains were in some ways extensions of things he encountered in the past. Malcolm Merlyn, Slade Wilson, and Ra's al Ghul were all connected to him, but Damien Darhk was a new villain who used magic and darkness. Oliver had to learn how to fight his own demons before he could use the power of the light to defeat him. With the overall tone of season 4 being lightened and the themes of light vs dark, this allowed Oliver to become a more inspirational hero instead of one people feared.
Running for mayor was the best thing Oliver could do. He saved the city twice already as the Arrow, but in season 4 he was ready to do so as Oliver Queen. Star City needed someone who wasn't afraid of the threats being a mayor could bring, considering the awful track record they had with past candidates. Oliver throwing his hat in the race showed he fully embraced the idea of fighting as a hero in the light, even going as far as giving an inspirational speech outside of his costume.
Oliver is responsible for jump-starting the careers of several vigilantes in the Arrowverse. And while he has really only trusted his friend Barry Allen, he showed a new kind of willingness to team up with other crimefighters in season 4. The Heroes Join Forces crossover, for example, marked the first time viewers saw Oliver leading a team of actual superheroes in battle, like a real hero. He had the most experience out of any one on both Team Arrow and Team Flash, so naturally he would fall into the leadership role.
Season 4 saw a necessary shift for Arrow and for Oliver. He was finally set up to mirror his other more heroic interpretations - choosing to defend his city rather than murder the people responsible for its suffering. He became a beacon of hope, even for those who originally opposed the idea of a masked vigilante. It also helped him gain the respect of the superhero community, as he would continue to lead them in future crossover events. And with his final act of bravery in Crisis on Infinite Earths (going as far as to atone for the lives he took), it's safe to say that Oliver Queen was nothing short of a hero.