Seo first became known to the public as the "4D Girl" (wacky girl) in the popular "Ok me wa ka" commercial promoting the Lotte ice cream brand. After her debut as a supporting actress in the 2007 film My Son, she successfully auditioned and landed the part in the sitcomKimchi Cheese Smile.
The following year, Seo received her breakout role in the black comedyCrush and Blush. She received considerable attention and the "Best New Actress" title in numerous awards for her success in the part—she was nominated for the same title in film at the 45th Baeksang Arts Awards.[4][5] In 2009, Seo took on the lead role in the cult favoriteTamra, the Island, a period drama set on Jeju Island.[6][7] She then contributed her vocals to "My Fair Lady," one of the tracks in Lee Seung-hwan's 20th anniversary album Hwantastic Friends.
She and Moon Geun-young played siblings in the popular modern fairytale retelling Cinderella's Stepsister,[13][14][15][16] but Seo later said she considers it her worst performance, despite being the character she worked hardest on.[17] Afterwards she played a complex character in the melodrama Flames of Desire.[18]
In the family drama If Tomorrow Comes, she played a cheerful daughter to veteran actress Go Doo-shim.[19][20] She next portrayed a calm yet brave art major in the fantasy-horror TV movie Knock, about a magical mask passed down in a shaman family.[21][22]