The strikes, involving hundreds of workers, will be a first for the charity in its 81-year history.
Members of Unite who work in Oxfam’s shops and offices will stage walkouts at its offices and more than 200 shops.
Why are Oxfam workers striking?
Workers will be striking in a bid for higher pay, which follows average wage cuts by 21 per cent since 2018, according to Unite.
Workers recently rejected a six per cent pay rise, or £1,750, in addition to a one-off payment of £1,000 for the lowest earners. They subsequently voted by 83 per cent in favour of strike action in a ballot with an 82 per cent turnout.
Unite general secretary Sharon Graham said: “Oxfam wants to end poverty and says it is on the side of unions. Yet its own workers report having to use food banks, it refuses to engage with the only union representing its workforce, and it is considering using unpaid labour to break a strike.
“This is rank hypocrisy from an organisation that should know better.
“Oxfam is an extremely wealthy organisation and can afford to put forward an acceptable offer without impacting its charity work in the slightest. Its workers have their union’s total and unflinching support as they strike to make sure that happens."
When are Oxfam workers striking?
Workers will strike on 17 non-consecutive days over the Christmas period beginning today, which will see union talks at Acas. Industrial action will take place on December 8, 9, 14, 15, 16, 17, 20, 21, 22, 23, 24, 26, 27, 28, 29, 30, and 31.
Unite warned industrial action will escalate if the dispute is not resolved. Meanwhile, the charity stressed it is doing everything it can to "minimise disruption and to support all colleagues".
A spokesperson said: “While we are disappointed that [today's] strike is going ahead, we do understand the frustration of colleagues who are facing a steeply rising cost of living.
“We are proud to be a Real Living Wage employer and are doing what we can to address colleagues’ concerns within the limits of the resources we have available. Communities we work with to fight poverty are also facing rising costs, so our resources are stretched but we are committed to finding a solution."
The row over strikes comes amid Oxfam GB's appointment of chief executive Halima Begum, who was previously CEO of the Runnymede Trust. She is to take over from Danny Sriskandarajah who will step down later this month.
According to Glassdoor, Oxfam directors' salaries range from £95,442 to £147,071 pa. Sriskandarajah was reportedly on nearly £121,000.