There are twenty-four fields (twelve before April 2007 and four prior to April 2002), oper_01 to oper_24, which contain information about a patient's operations. The field oper_01 contains the main (ie most resource intensive) procedure. The other fields contain secondary procedures.

The codes are defined in the Tabular List of the Classification of Surgical Operations and Procedures. The current version is OPCS4. Procedure codes start with a letter and are followed by two or three digits. The third digit identifies variations on a main procedure code containing two digits. The third digit is preceded by a full stop in OPCS4, but this is not stored in the field.

A single operation may contain more than one procedure.

annn = Procedure code

- = No operation performed

& = Not known