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The Korean-Canadian actress, 46, was honored for her role as British spy Eve Polastri on the BBC series "Killing Eve." After hearing the news, Oh took to Twitter with a photo of herself — looking adorably shocked — beside Taiwanese-American actress Michelle Krusiec.
"Omg wtf?? This is me hearing about Emmy nom w/ fellow sister @michellekrusiec — my expression is kinda ... what?" she wrote.
While Oh's fans and colleagues are cheering her milestone nomination, no one's more excited than her former "Grey's Anatomy" co-stars.
Ellen Pompeo, who's played Dr. Meredith Grey on the ABC series since its 2005 debut, evidently missed Oh's big news on Thursday, but early Friday morning she was jumping for joy for her one-time cast mate.
Yes!!! So incredibly happy for @IamSandraOh, who deserves this more than anyone," Pompeo tweeted. "No one can touch this girl's dedication to storytelling or her talent."
Justin Chambers, who plays Dr. Alex Karev, shared a news story about Oh's historic nomination with his Twitter fans — which Pompeo excitedly retweeted while gushing over Oh a little more.
"I was not on my phone yesterday and clearly missed A LOT!!! Congratulations @IamSandraOh. This is beyond well deserved and your history making is incredibly moving. Can't wait to hear your speech!! Which will have us in tears and laughing at the same time no I'm sure," Pompeo wrote.
Kevin McKidd, who played Oh's TV hubby, Dr. Owen Hunt, also toasted the actress on Twitter, writing, "Amazing!!!!! So much love! Xoxo."
"Our hearts are also bursting with pride and excitement for Sandra Oh, making history as the first woman of Asian descent to be nominated for Lead Actress in a Drama Series!," it read.
While it's Oh's first nomination as a lead actress, she previously scored five supporting actress nominations during her decade-long run as Dr. Cristina Yang on "Grey's" before leaving the series in 2014.