Ohila was the High Priestess of the Sisterhood of Karn, and was active during the Last Great Time War, as well as the aftermath of the war's end.
She was pleased to see the Eighth Doctor return to Karn, but pitied the fact he had been killed in a crash landing, following his inability to save Cass because she refused his help, owing to her hatred of the Time Lords. The Sisterhood revived him for four minutes, and Ohila begged the Doctor to help them by taking the Elixir of Life. After arguing with him, she convinced the Doctor that he was the only one capable of stopping the war, and he could not help anyone by running away from it just to uphold his principles as the Doctor.
Once the Sisterhood retrieved Cass's body from the wreckage, the Doctor gave up hope and decided to enter the Time War. He used her potion containing the Elixir of Life to regenerate into a warrior for battle, no longer going by the name of "Doctor". (TV: The Night of the Doctor)
Ohila met the Doctor again in his twelfth incarnation. He was present on Karn when Colony Sarff arrived and spoke to Ohila about searching for him on behalf of Davros. Sarff gave a message to Ohila to pass on to the Doctor (unaware he was listening), stating that "Davros remembers", and an invitation to meet him one last time. (TV: The Magician's Apprentice)
She didn't believe his answers when discussing the invitation, stating that it was easier to assume he was lying to her rather than trust his word. She also indicated that she considered herself and the Doctor to be friends, an assertion he didn't refute. Before leaving Karn, the Doctor gave her his confession dial, saying that she'd know who to give it to. (WC: Prologue) She later gave it to Missy. (TV: The Magician's Apprentice)
When Ohila heard the Twelfth Doctor had returned to Gallifrey, she went there and met with the General and the High Council, much to the Lord President's irritation. Ohila watched Rassilon's efforts to reach out to the Doctor with amusement and told Rassilon that the Doctor didn't blame Gallifrey for the horrors of the Time War, but rather Rassilon himself. After Rassilon was overthrown, Ohila spoke with the Doctor about his actions and about the Hybrid. She later consulted with the General about the Doctor's actions and, deep in the Cloisters, told Clara what happened in the Doctor's confession dial and why. As the Doctor and Clara escaped in a stolen TARDIS, Ohila yelled that he was going against everything he ever believed in before leaving, disgusted. The Doctor ultimately acknowledged that she was right. (TV: Hell Bent)
Behind the scenes
According to an interview with Steven Moffat, Ohila's name is a reference to Ohica.[1]