The societies said that they would seek support for the merger from their combined 175,000 members in July.
If approved, the merger would create the fourth largest co-operative in the UK, with a turnover of £500m, over 350 stores and 7,000 employees.
Both their offices in Oxford and Walsall would be retained, although there was no guarantee that jobs would not be affected, the societies said.
OS&G’s chief executive, Bob Burlton, will lead the new society until he steps down in the second half of 2006. Ben Reid of West Midlands Co-op will be appointed chief executive designate and will take over as chief executive in 2006.