Armstrong was the first chief executive at Oath, the name chosen when Verizon acquired Yahoo and incorporated the internet pioneer into a new division that had promised to innovate in online media.
Verizon said in a statement that Armstrong "will help guide the Verizon subsidiary's management transition efforts as a strategic advisor before leaving the company at the end of 2018."
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He will be replaced by Guru Gowrappan, who has served as Oath's president and COO since April.
Gowrappan, a former Alibaba executive, "has proven experience in scaling businesses globally," said Verizon CEO Hans Vestberg.
"I'm thrilled he will lead Oath in an exciting new phase of growth, building on the foundation Tim and his team have created by delivering brands our customers love."
Armstrong served as CEO at AOL from 2009 until it was acquired by Verizon in 2015. He steered the group into online media, acquiring properties including the Huffington Post and TechCrunch.
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Combining AOL and Yahoo was seen as a way for Verizon to move deeper into online media and digital advertising.
At the time of the acquisition of Yahoo, Armstrong tweeted: "Billion+ Consumers, 20+ Brands, Unstoppable Team. #TakeTheOath. Summer 2017."
But the rebranding was mocked on social media, with one Twitter user writing: "Q: Is "Oath" Yahoo or AOL? A: Boath."
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