Slocombe has put pen to paper on a two-year contract with the U’s and becomes the fourth new arrival at the League Two clubs this summer after the acquisition of midfielders Kemar Roofe and Liam Sercombe and striker Ryan Taylor.
“Sam is a proven goalkeeper at this level and above and we are delighted to have him here,” said Oxford boss Michael Appleton.
“He communicates with his defence and he is an excellent shot-stopper. He is still only 27 so we think there is plenty to come from him.
“It gives us serious competition for that one position and that is something we feel we will have all the way through the team next season.”
Slocombe featured just 15 times last season for Scunthorpe after joining his hometown club in 2008.
He helped the Iron win promotion from League Two in 2014 and believes he can play a similar role for the U’s.
“The manager doesn’t just want to go up a division and stabilise, he has a long-term plan that will increase the club’s popularity and what it does within the divisions,” said Slocombe.
“It’s down to me to come in, work hard in pre-season and hopefully be part of a successful squad that challenges at the top end of the table.”