The bid to put up the 12.5m monopole on land adjacent to Welford Road, Kingsthorpe, was originally turned down by Northampton Borough Council in June.
But O2 appealed and an independent planning inspectorate in Bristol overturned the decision.
Irene Rose, aged 76, of Larwood Close, off Welford Road, spearheaded the campaign and helped present a 65-signature petition to the borough council. She said: "I feel the general public don't count; our opinions on all sorts of things are just brushed aside.
"O2 had looked at privately-owned land and the landowners said 'no'. Well, Northampton Borough Council is a landowner, and we pay our council tax to be part of that."
Mrs Rose said nearby residents were opposed to the monopole because they were concerned about the health implications and the visual pollution of the area.
She said: "There is not enough proof to say that there is no danger to health. We have got the local playgroup nearby and the fields near it are used for sport.
In the report on his decision, planning inspector Ian Radcliffe wrote: "The appearance of the street scene in the vicinity of the appeal site is that of an active residential area with pedestrian islands, railings, bus stops and street lighting. The three existing telecom poles add to the street furniture.
"There is nothing to indicate that there would be an actual risk to health."