In total there are 317 listed buildings in the city of Southampton, of which 14 are Grade I, 20 are Grade II* and the remainder Grade II.
In England, a building or structure is defined as "listed" when it is placed on a statutory register of buildings of "special architectural or historic interest" by the Secretary of State for Culture, Media and Sport, a Government department, in accordance with the Planning (Listed Buildings and Conservation Areas) Act 1990. Historic England, a non-departmental public body, acts as an agency of this department to administer the process and advise the department on relevant issues. There are three grades of listing status. The Grade II designation is the lowest, and is used for "nationally important buildings of special interest". Grade II* is used for "particularly important buildings of more than special interest", while Grade I (defined as being of "exceptional interest" and greater than national importance), is the highest of the three grades.
This list summarises 35 Grade II-listed buildings and structures whose names begin with M to O. Numbered buildings with no individual name are listed by the name of the street on which they stand.