During the opening ceremony of BlizzCon 2017, Moira was introduced as Overwatch's 26th playable support class hero. With her addition to the game, a lot of unanswered stories was answered and more mysteries surfaced. Let's see what kind of impact did Moira bring to fill in the Overwatch lore.
Moira O'deorain is a Talon geneticist, who is also formerly affiliated with Blackwatch. Over a decade ago, she published her scientific paper regarding DNA alteration to cure diseases and disorders, and also maximize human potential. But little did she know, her paper ruined her own reputation. Moira was recruited into Blackwatch, Overwatch's covert ops division shortly after that, but Overwatch did not acknowledge her affiliation with them. After Overwatch was disbanded, she joined a scientific collective that built Oasis, and rumors of Talon supporting her for the organization's own good has been spreading.
When Overwatch was initiated, Jack Morrison and Gabriel Reyes joined the program at the same time, but Morrison was appointed as the first Strike Commander and Reyes was passed over. Due to this, their relationship was tense and it was getting worse as days gone by. Dr. Angela Ziegler, also known as Mercy, was close to both of them, she tried her best to mend it but it didn't prevail.
An unfortunate incident happened later on at the Overwatch Swiss Headquarters where the building was destroyed in an explosion. Along with it, Morrison and Reyes were announced to be dead as both of them were in the Headquarters when it happened and Overwatch has shut down shortly after. A popular fan theory says that Mercy was responsible for resurrecting both of them, whereby Morrison became Soldier:76 and Reyes turned into Reaper, however, it turns out that is not the case.
As we can see in her Origin Story video, Moira has her hands on Reyes and as the video plays on, his face shown disbelief as his body started reacting to it, which turned him into the Reaper we all know now. That scene itself was enough to debunk the fan theory that Mercy was the one who is responsible for resurrecting Reyes in a permanent state of death and recovery, as his cells were decaying and reviving at an incredible rate.
Blizzard also teased us about the relationship between Moira and Mercy. An Overwatch player was patient enough to dig through all of Moira's cosmetics and compare it to Mercy's where both of them have matching sprays.
So far there is no interaction between Moira and Mercy in game, but there is one with Widowmaker that could probably mean that Moira is the one responsible in brain-washing and turning her into a cold-blooded assassin where she even killed her own husband. During the interaction, Moira asks Widowmaker: "How are you feeling, Lacroix?", and she will reply: "I don't feel. That's the point, isn't it?" There is no evidence that she is the culprit but that is the best guess for now.
Even though Moira's addition has answered some questions about the Overwatch lore, a new one will surface as it will still unfold many other mysteries, unless Blizzard decides to spill it all out, hopefully.