Shah Jahan Khan has become the fourth Liberal Democrat city councillor to quit the group in two years.
Earlier today Mr Kahn, the Cowley councillor, defected to Labour in a major coup that now leaves his new party the largest group on the council with 19 seats.
The ruling Liberal Democrat administration has 15 seats, the Greens eight, Independent Working Class Association four and Conservatives two.
Married father-of-three Mr Khan, 52, said he was "unhappy with the way they (Lib Dems) run things" and added "I know there are quite a few people who are not happy".
Flanked by Labour's Oxford East MP Andrew Smith and city council group leader Bob Price at a hastily-convened meeting this morning Mr Khan said: "Over the past few months I have become very unhappy with the lack of direction with the Liberal Democrat administration.
"They are out of touch with the people in east Oxford and have handled the introduction of the recycling scheme, which all parties on the council supported, in a disastrously inflexible and insensitive way."
Mr Khan also blamed a so-called "north Oxford bias" as another frustration, claiming the Lib Dems favoured that area as opposed to the south and east of the city and blasted their handling of the ongoing play area closure crisis.
Since the Lib Dems took control of the city council in May, 2005 four councillors have quit the group.
First Saj Malik left for Labour then Tia MacGregor and Paul Sargent left to sit as independent councillors before forming a Tory group.
However, Liberal Democrat group leader John Goddard insisted there was no crisis.
He said: "It's regrettable but in terms of numbers it means nothing at all.
"I feel sad these people haven't been more open sooner - that is a personal failing.
"We are quite a friendly group and we are open with each other.
"Every one of these people has chosen to leave without any warning whatsoever."
News of Mr Khan's defection will come as a fillip to Labour ahead of next year's city council elections when half the 48 seats are up for grabs - including Mrs MacGregor's Quarry & Risinghurst seat and Mr Sargent's Carfax ward.
Labour might be the largest group on the council, but cannot take control until September at the earliest when the next full council meeting is held.
Mr Price said: "Shah is well-known as a highly effective councillor for Cowley and he will be made very welcome by Labour members and councillors."
Mr Smith added: "I have known Shah for years, his choice to join Labour is great news and reflects a wider shift to Labour since Gordon Brown became Prime Minister."