Pros
Inspiring place to work; your effort matters and will directly benefit others. Colleagues are motivated, hard working, knowledgeable and mostly friendly. The mix of age groups, nationalities and cultures is very interesting to be part of. Work content is varied and stimulating, you get to see the big picture and the details. Scope to travel to and/or learn about issues and places you'd never know about otherwise. Oxford offers a good - but not cheap - quality of life.
Cons
Induction is prefunctory given there is a massive amount to learn in this complex organisation. Huge bureaucracy, loads of administrative flim-flam, hard to feel effective in your role. Inefficiency, training / intro to Oxfam courses not happening until long after they'd be useful. Continual interruptions to attend briefings and meetings of little tangible benefit, though they did have the upside of not spending the entire day glued to a computer screen. Lot of silos, surprisingly little interaction between different teams/depts/confederation members. Surprisingly high staff turnover, particularly at senior management level. Horrible location on the ring road, no where to go at lunch except staff canteen. Poor payers compared to their peers.