Plum Island Point’s last surviving “old hotel” will be torn down following approval last month of a demolition permit by the Historical Commission. The Oliver House, at 245 Northern Boulevard, will be razed for safety reasons, according to city officials. The exterior of the vacant, 13-room structure is sagging due to structural problems. The Oliver House opened about 1900 as a guesthouse and hotel. Over the years it has fallen into disrepair and it has been empty for several decades. The Historical Commission’s vote granting the demolition permit was unanimous. It is not clear when the structure will be demolished.