The transaction, which puts a price tag of about $12 on each of Oxygen's subscribers, is expected to close by next month.
Oxygen was founded by Winfrey in 2000 with former Nickelodeon TV executive Geraldine Laybourne, Oxygen's current chairman and chief executive. It broadcasts a mixture of female-centric comedy and reality shows to more than 74m homes.
It is the second cable acquisition made by NBC since new chief executive Jeff Zucker was appointed earlier this year -- signalling a move away from its network television heritage. In August, NBC paid $350m for Sparrowhawk, the owner of the Hallmark Channel in the UK, Australia and Italy.
Zucker said: "This acquisition [of Oxygen] increases our foothold in the advertiser-coveted young, upscale, female demographic. It perfectly complements our current roster of cable channels."
Oxygen will be added to NBC's existing cable TV assets, including Bravo, the Sci-Fi Channel, financial broadcaster CNBC and breaking news channel MSNBC.