As always the Shots are stating that he has moved for "an undisclosed fee" which is being described as "substantial" by the U's website.
Sills joins the Oxfordshire club on an 18 month contract with an option to extend but will not play any part in their match against Rushden & Diamonds tomorrow and will begin training with his new club on Monday morning.
The U's are having a fairly poor season and are in 17th place - managed by former Shots (1926 Ltd) boss Brian Talbot.