It is hard to imagine why anyone would have moved from the UK's international aid leviathan to such an obscure body. Admittedly, Opportunity International is only one branch of a charity with offices in the US, Australia, Canada and Germany, but its development model is very different from that of Fox's previous employer.
Oxfam provides traditional overseas hand-outs and development aid on a titanic scale; Opportunity International arranges loans to help countries work themselves out of poverty through entrepreneurship.
Fox's move was motivated by more than becoming a chief executive. "The organisation is tiny," he says. "I looked at what it does and how it does it, and at the potential for it to grow - not just by 50 per cent, but 10-fold. I like the idea of taking something that I believe in and taking it from a £1m or £2m turnover to £10m or £15m."
As a marketing expert, Fox felt that he had little left to achieve at Oxfam. "My team took the charity from an income of £76m to one of £111m, an increase of more than 60 per cent in three years," he explains. "I took us from being the third biggest charity to the second biggest."
Under Fox, Oxfam won the Direct Marketing Association's gold award for its Oxfam Unwrapped campaign in 2006. "I felt there wasn't any further to go from there," he says.
His job change was also spurred by an altogether more private motivation.
"I'm never 100 per cent sure about these things, but I think that God wanted it," he says. "You can never say it in a way that sounds remotely normal, but this job was an opportunity for me to follow what felt like a calling."
Opportunity International has religious affiliations itself: it professes to be a response to Christ's call to serve the poor. Working in 28 countries in Africa, Asia, South America and Europe, it helps the impoverished with business loans, bank accounts, savings schemes and insurance.
When asked if the move presents him with a chance to embrace a more modern development model, Fox refuses to criticise Oxfam's style. "You won't get a thing out of me that says anything bad about the way that Oxfam operates," he says. "I'm very proud to have worked there."
But he does make concessions to the change of method. "We believe in a hand-up, not a hand-out," he says. "This is about poverty eradication rather than wealth creation, taking things up from the grass-roots, family by family. There's a big connection between what you do and how people benefit. We work to preserve people's dignity - they don't want to be dependent."
The charity refers to the people it helps as clients, rather than beneficiaries.
"There is a business transaction going on here, human being to human being," Fox explains. "There's a problem in the aid community of people being labelled as victims."
Fox is enthusiastic about making the charity into a bigger, better-known organisation and capturing new sources of income. "Our biggest growth will come from individual major donors," he says. "There might be people who are cynical about traditional charities, or people who have never engaged with charity before because they don't think it works."
Only a month into the job, Fox has already set high standards for himself and his team. "If we don't succeed, it will be because we haven't made the proposition clear enough in the right people's minds," he says.
2007 - Chief executive, Opportunity International UK
2003 - Fundraising and supporter marketing director, Oxfam
1999 - Marketing director, World Vision
1992 - Head of marketing, Anglian Water
1985 - Marketing controller, United Biscuits