Mick O'Brien and Doug Chapman are leaving Allen & Gerritsen. The two ran the Watertown, Mass., agency's creative department for a decade before being supplanted following an acquisition last June. O'Brien last week joined Digitas, Boston, as an svp, group creative director. Chapman will move to IPG's Hill, Holliday, Connors, Cosmopulos, Boston, as a vp, associate creative director. Both execs and A&G senior management sought to distance the defections from the shop's acquisition of GSO/Davis. Gary Greenberg and Peter Seronick, GSO/Davis principals, were made evps, cds of the combined agency following the purchase.