Nedar, known as Oh while posing as a Starfleet officer, was a female Romulan/Vulcan hybrid flag officer in the Tal Shiar with the rank of general, loyal to the Zhat Vash, and part of the Conclave of Eight.
She became integrated into Starfleet as a mole, where she worked her way up through the ranks to eventually become a commodore, and served as the director of Starfleet Security from at least 2390 to 2399.
In 2385, Oh presided over a Zhat Vash initiation ceremony on Aia, of which Narissa and Ramdha were the only survivors. She was one of the architects of the the devastating Attack on Mars the same year. (PIC: "Broken Pieces")
In 2390, Oh issued a black flag directive to Captain Alonzo Vandermeer of the USS ibn Majid, ordering him to kill Ambassador Beautiful Flower and his protégé Jana, or risk the destruction of his ship. (PIC: "Broken Pieces", "Et in Arcadia Ego, Part 1")
Oh ran covert operations involving Romulan operatives Rizzo and her brother Narek, among others, aimed at obtaining information about the whereabouts of androids similar to Dahj and Soji Asha. According to Rizzo, she had profound reservations. At least one of these operations was inside the Federation on Earth. (PIC: "Maps and Legends")
After that operation resulted in the death of Dahj and Picard becoming aware of the androids, she went to see Dr. Agnes Jurati at the Daystrom Institute in Okinawa to discuss the doctor's meetings with Picard. (PIC: "The End is the Beginning") There, Oh initiated a mind meld with Jurati showing her conflated visions and images related to the supposed threat of artificial lifeforms that Oh herself experienced during her exposure to the Admonition on Aia (PIC: "Broken Pieces"), before convincing Jurati to swallow a bio-organic tracking compound called viridium. (PIC: "Nepenthe")
Abandoning her Starfleet cover, Oh led a Tal Shiar Warbird squadron to Coppelius to destroy the synths living there, but encountered orchids and, eventually, a fleet of Starfleet Inquiry-class starships led by the USS Zheng He. Initially unswayed by Acting Captain William T. Riker's demand to retreat because she felt her goal to defeat the Destroyer was more important than her own survival, Oh eventually fled when Soji destroyed the beacon, proving she was not the prophesied Destroyer. (PIC: "Et in Arcadia Ego, Part 1", "Et in Arcadia Ego, Part 2")
"Picard again? And we all thought he was safely mothballed on that vineyard of his. All these years. The Hermit of La Barre. Now suddenly he is inescapable."
"That's one word for what he is."
"Admiral Clancy, I know I need hardly remind you if the Romulans were running clandestine operations on Earth, I would know. And then you would know. And then all of Starfleet would know, because clearly, that would be an overt and intolerable act of war."
"Do not insult me, lieutenant; it would not be advantageous for your career."
Oh was portrayed by Tamlyn Tomita. An alternate name, "Nedar", was given by closed captioning in the season 1 finale, "Et in Arcadia Ego, Part 2", but was not spoken on screen. "Nedar" was stated by Michael Chabon to be her real name, and "Oh" was her "sleeper agent name." 
She is the first commodore to be seen in the 24th century.
In "The End is the Beginning", Oh is shown wearing sunglasses. This was specified in the script as a homage to Danish-French actress Anna Karina.  It was previously established by T'Pol in "The Forge" that Vulcans don't need eye protection due to the existence of an inner eyelid.
On that account, Michael Chabon said the following on this post's comment section: "Let me try to be clearer: Oh has read Jurati's psychological profile, and something in it, a tendency in Jurati's relationship to authority figures, has led her to prepare both her manner and her appearance very carefully [….] This is behavior that cops, soldiers, and teenagers, among others, engage in all the time."
A line in "Broken Pieces" firmly established Oh as being the first featured onscreen individual of Vulcan/Romulan descent. The script of Star Trek II: The Wrath of Khan had already included an introductory note that Saavik was intended to have also been of similar descent, (; Star Trek Magazine issue 155, p. 62) but the reference did not make it onscreen ultimately. A Scene 5 line establishing the fact was actually filmed, but was trimmed from the scene as eventually featured in the released film, and had not been included – neither as a reinserted scene nor as a "deleted scene" special feature – in the later-released "Director's Edition/Cut" home video formats either. The line was actually included, though, in an early contemporary promotional short, aired on television.  This was also mentioned in the novelization of Star Trek: The Wrath of Khan .