The Australia Under-20 international has joined the U's on a deal until January and was an unused substitute for Saturday's 4-1 League Two victory away to Portsmouth.
Williams began his career in England with Pompey before switching to Fulham in January 2012. Last season he was sent on loan to Gillingham but failed to make an appearance under Martin Allen.
In the summer, the 19-year-old represented his country at the U20 World Cup in Turkey and then played for Oxford in pre-season before agreeing terms.
Wilder told the club website: "Ryan is a good player and will give us competition in an important area.
"He has a good pedigree, has pace, and is very highly thought of at Fulham, a club we have an excellent relationship with. He has a really good attitude and I am sure will do very well here."