The 77-year-old – half-sister of legendary folk singer Pete Seeger – told a meeting of the Labour-run authority she had left the party for the third time and joined the Greens.
At a meeting last week she said: “It is because of this council’s attitude towards, and its duplicitous relationship on such issues, that I have decided to come here today.”
The council wants to build a multi-million pound leisure facility in Blackbird Leys.
Deputy leader Ed Turner said: “The Government is savaging living conditions of people on low and middle incomes. It is an odd issue to be focusing on.”