The legal bit
In keeping with charity law, please include the following statements across all publicity materials:
All proceeds/profits will be donated to Oxfam. Oxfam GB is a member of Oxfam International, registered as a charity in England and Wales (no 202918) and Scotland (SC039042). Oxfam works with others to overcome poverty and suffering.
It's important you make it clear you are raising money in aid of Oxfam. If you're not an Oxfam employee, please note somewhere that you don't represent the organisation in a work capacity.