More than 1,600 rural post offices could be shut down under new regulatory recommendations.
The latest walkout affects the OX1 and OX2 postcode areas
The postal services commission PostComm says the £150m-a-year subsidy which sustains the rural network of post offices is too much.
The review is now being considered by the government.
The Department of Trade and Industry said the post office network could be run with fewer branches but a decision had not yet been made.
A spokeswoman said: "It could be run with 1,600 less but that's just the regulator's view.
"We've seen the report and we've been noting their views but any decision is a matter for Royal Mail and the government."
The report by the independent regulator into the long term future of the post offices has been with the government since July last year, but the DTI says there are no immediate plans to publish it.
The spokeswoman added: "PostComm delivered private advice to the government at the end of July 2003 which included a recommendation about the size of the rural network.
"PostComm also recommended that the government's current policy of no unavoidable closures should continue until there is a wider strategy to ensure delivery of services in rural areas.
"The government is committed to the future of the rural network and has put in place funding of £150m a year to sustain the rural network of post offices until at least April 2006."
Post office survey
59% say it's "essential to their way of life"
75% say it's "extremely important" to them
91% say it "plays an important role in their community"
The subsidy, known as the social network payment, was introduced by the government in December 2002 following earlier recommendations made by PostComm.
Post office closures are a politically contentious issues, with independent research from Postwatch, a postal watchdog group, showing three quarters of respondents felt that their local post office was "extremely important" to them.
More than half, 59%, thought it was "essential to their way of life" and 91% agreed that it played "an important role in their local community".
A spokesman from PostComm said the review of the long term future of the post offices was "substantial" but declined to offer further comment.