The Orthodox celebrate her feast day on July 11, but she shows up today with her grandson Vladimir (who was killed on this day) in the LBW
It was her grandson Vladimir who is responsible for converting Russia to Orthodox Christianity but it was certainly Olga who planted the seeds.
Olga was born in 890 to a Viking family . She married Prince Igor who was the son of Rurik known as the founder of Russia and they ruled Kiev which is made up of the modern day nations of Russia, Ukraine, Belarus and Poland.
In 945 , when Igor was murdered by a slavic tribe who got tired of paying tribute, she became regent for her son. They sent messengers suggesting that Olga marry their prince. She declined and killed the messengers. She had 20 matchmakers who had been sent buried alive. She asked for better ambassadors and when they showed up she scalded them to death in a bathhouse. She went to the land of the people responsible for killing her husband for a feast in memory of her husband, got them all drunk and had them killed, and burned down their town. Do not mess with Olga! She was an effective, if ruthless ruler.
Sometime after her son came of age and took over as ruler she became interested in Christianity and visited Constantinople. It is said the emperor Constantine was quite taken with her and proposed marriage. She was baptized and asked Constantine to be her godfather and afterwords said she couldn't possibly marry him because she was like a daughter to him.
There is nothing about what led her to embrace Christianity. Perhaps all the vengeance and violence brought her no peace and satisfaction for her husband's murder. Perhaps the innocent lives she took lay on her conscience and in the end it was only the promise of grace and forgiveness that could bring her peace. Or perhaps it was as is told that when she saw the splendor of the Hagia Sophia Basilica she figured they must have the best god.
Olga never was able to persuade her son to convert to Christianity but he was not against the new religion and did not hinder anyone else from converting. And you have to believe she was a significant influence on her grandson Vladimir who did become a Christian and was responsible for bringing Orthodox Christianity to Russian. In 1547 the Orthodox Church declared her to be a saint equal to the Apostles.