A banner promoting a family fun day in aid of a children's playground was stolen a week before the event.
It was taken from railings around Cowley Recreation Ground, off High Road, Cowley.
The 4.5metre by 1metre sign cost £170, paid for by the three mothers who hope to raise £75,000 to build a multi-purpose games area for their children in the recreation ground.
Mother-of-four Emma Driscoll, of Barlee Crescent, Cowley, said: "I was so angry when I found out. I couldn't believe some low-life would stoop so low to do that. It's devastating."
Featured in the Gazette last month, Mrs Driscoll andfriends Mandy Newnham and Anita Perry, want to raise the cash after repeated clashes between children playing in the streets and the area's Safer Neighbourhood Teams.
The women hope to create a facility which will cater for teenagers and anyone with a disability, to give young people in the area somewhere safe to go.
But the friends are now devastated that vandals have attempted to squash their community spirit.
Mrs Driscoll said: "We've looked all over the park and nobody has seen it. I can only think someone has taken it and repainted it.
"It's terrible, especially seeing it's the last week.
"It's like one big slap in our faces." Mrs Driscoll believes the banner was taken during Saturday night and has reported the incident to police.
Now he mothers still plan to go ahead with the family fun day on Sunday, at the recreation ground from noon-4pm. There will be a barbecue, pony rides, car boot sale and a football tournament.
Mrs Driscoll added: "They've basically stolen from a charity. It goes against everything we are trying to achieve.
"If I find out who has done this then I'll go mad.
"We won't let it ruin the day."