THE place-name Cathcart is derived from the p-Celtic or Brythonic (ancestor of the Welsh and Breton languages) ’Caer Cart’.Caer means a fort, and Cart is the name of the river which runs through the parish of Cathcart.
This parish was given to a Breton knight named Rainald (who took the name Cathcart) in the early 12th century. Rainald was probably an associate of Walter Fitzalan, also a Breton, who was allocated lands in Renfrewshire and Ayrshire. The new owner built a castle at the south end of what is now Old Castle Road, and a mill below the castle. The parish church was on a hillock to the north. Its site was probably used for worship before that time.
The church was dedicated to St Oswald, a 7th-century Northumbrian saint.