Edward McKenzie-Green, 34, pled guilty at the Old Bailey to making £64,612.58 in payments from Oxfam to fictitious firms between February and December 2011.
A charge against his father for allegedly helping him to launder more than £35,500 was dropped.
Edward Green, 62, of Cumbernauld, North Lanarkshire, was accused of moving £35,571.75 from his bank account into a Santander bank account in the name of Edward McKenzie-Green by electronic transfer.
Mr Green, who was excused from attending court, denied a charge of transferring criminal property between April 1 2011 and March 31 2012.
McKenzie-Green, former head of counter fraud at Oxfam, was also accused of stealing a laptop from the charity in December 2011 but the charge was dropped.
Matthew Sherratt, defending McKenzie-Green, told the court that up to January last year his client had a "serious addiction to prescription medication".
"At the same time his marriage, which he was attempting to hold together, was failing," the barrister said. "And he was involved in quite a high-pressured job. The combination meant he effectively suffered a breakdown."
McKenzie-Green, from Chipp-ing Norton, Oxfordshire, was admitted to a psychiatric hospital in January last year when doctors noted his "erratic behaviour", Mr Sherratt added.
The defendant committed the fraud following a "false sense of victimisation" which then escalated when he was put on gardening leave by Oxfam, Mr Sherratt said.
"Some behaviour appears to be paranoid," he added.
Prosecutor Adam King said McKenzie-Green's scam involved submitting false invoices to companies he had set up, paying money into the bank account of his friend or father, before transferring the cash into his own account and spending it.
"It was certainly carefully planned and real thought went into it," Mr King said.
"Even afterwards, when he thought the game was up, he attempted to cover this."
Mr King said McKenzie-Green used a fictitious name, Keith Prowse, and created fake companies including Solutions de Recherche Intelligence.
But "certain aspects" of the scam were "very unsophisticated", the prosecutor said.
This included the defendant misspelling his false company's name which would "cause people to smell a rat", Mr King said.
McKenzie-Green also claimed to have had high-powered jobs with Carphone Warehouse and Marks & Spencer, where he says he worked closely with police in stopping shop theft, fraud and violence.
The court heard he was of previous good character and is joint carer to a young child.
He was released on conditional bail as Judge Timothy Pontius adjourned sentencing to May 16.
"A prison sentence will be at the forefront of the court's mind," the judge said.
Following the hearing, Oxfam's head of counter fraud Oliver May said: "Oxfam uses donations from the British people to help eradicate poverty and suffering across the world.
"For someone to commit fraud and use that money for their own gain is clearly unacceptable. The actions of Edward McKenzie-Green were brought to light by Oxfam's own robust counter fraud measures.
"We work tirelessly to ensure money donated to Oxfam is not lost through the actions of rogue employees like McKenzie-Green. We will now work to try and recover the money taken."