The 33-year-old spent just one season at Layer Road, having joined the Us 12 months ago from Crystal Palace.
The Os boss Martin Ling said: "It is pleasing the get the first summer signing done and dusted, especially one of Danny's quality.
"Danny is primarily a left-back, but he has also played left-sided centre-half, which is important to me as it gives us four quality centre-halves and two quality left-backs to choose from.
"I think it shows our intentions for next year by bringing in a player of Danny's calibre."