The society, which each year gives out awards for good design in architecture and building, has objected to a planning application for about 50 homes on land to the north of Whittington Road in the town.
It is the latest to object to the proposals, which have been submitted to Shropshire Council by Galliers Homes.
The application site, which is about 450 metres from the hillfort, was zoned as housing land by Shropshire Council and ratified by planning inspector in a SAMDev inquiry, looking at future development in the county.
David Ward, spokesman for the Oswestry and District Civic Society said members had been kept abreast of the developments on the proposal.
"It has been our view that, whilst the conclusions of the SAMDev Inquiry were unfortunate, and based on too short-term a view, nevertheless a democratic process had been followed, and the issues of that time should not be revisited.
"However, on review of the documentation, the Society is of the view that the proposals now presented do not meet the conditions under which development in the locality was found acceptable by the Inspector at the time of SAMDev. Furthermore, examination of the details by those with appropriate expertise demonstrates clear conflict with the policies of the National Planning Policy Framework for the protection of the Historic Environment.
"In relation to the details of the proposed development, the society considers that the layout proposed and the design of house types is at best mediocre, and at worst atrocious. The proposals should be judged carefully against the newly published National Design Guides, which elaborate upon the requirements of the framework for there to be good design."
Mr Ward said that even if the planning authority considers that the Historic Environment is adequately protected by these proposals, they should nevertheless be refused on the grounds of conflict with the framework.
He said the Society was objecting to the proposals.
A Heritage Impact Assessment commissioned by the developers said: "The proposed development will cause less than substantial harm to the setting of Old Oswestry Hillfort and will have little or no impacts on the setting of the other assets."