Acquired off waivers on September 6th, O'Callaghan came from the New England Patriots, formerly the home of GM Scott Pioli. O'Callaghan's rise to starter is no surprise - I think that was the plan eventually. But what did surprise me is how quickly it came. Previous starter Ikechuku Ndukwe, acquired prior to the start of the season as well, has been relegated to second team duty.
There's also a new starter at nose tackle: Ron Edwards
Other changes include:
H/T GenericBrand and DThomasReigns in the FanShots for the link