Gytha Ogg (usually called Nanny Ogg) is a character from Terry Pratchett's Discworld series. She is a witch and member of the Lancre coven.

The character of Nanny Ogg is based on the Mother stereotype of the Triple Goddess myth. She's had 5 husbands and been married to three of them and has fifteen children who survived their early childhood (one was born some ten years after the death of her last husband). She has as well innumerable grandchildren and great-grandchildren, but that's incidental; what makes her the Mother is her mentality. People go to Granny Weatherwax for help when they have no choice, but they go to Nanny for advice all the time. Granny is respected, but Nanny is actually liked.

Nanny Ogg has a talent for getting along with people and fitting in. As described in Maskerade, people, after knowing her for fifteen minutes, feel like they have known her all of their lives. Granny Weatherwax knows about this ability, and recognizes its use, and wonders sometimes if it would have been worth acquiring it.

She is wiser than Esme Weatherwax in some ways, and certainly wise enough not to show it. Nanny Ogg is seen as "one of the people" in a way that Esme isn't. While Granny thinks there's no point in competing if you aren't going to win, Nanny reckons the sympathy you get for being a good runner-up is much better. Granny comes across as judgemental, whereas Nanny has a mind so broad she could tie it under her chin. She appears to be kinder than Granny, but is equally prepared to make tough decisions if necessary.