The report is one in a six-volume series considering abnormal operating conditions (AOCs) in the primary section (sintering, blast furnace ironmaking, open hearth, electric furnace, and basic oxygen steelmaking) of an integrated iron and steel plant. Pollution standards, generally based on controlling discharges during normal (steady-state) operation of a process and control system, are often exceeded during upsets in operation. Such periods of abnormal operation are becoming recognized as contributing to excess air emissions and water discharges. In general, an AOC includes process and control equipment startup and shutdown, substantial variations in operating practice and process variables, and outages for maintenance. The purpose of this volume, which covers the basic oxygen process, is to alert those who deal with environmental problems on a day-to-day basis to the potential problems caused by AOCs, to assist in determining the extent of the problems is a specific plant, and to help evaluate efforts to reduce or eliminate the problems. The report enumerates as many AOCs as possible, with emphasis on those which have the most severe environmental impact. Descriptions include flow diagrams, material balances, operating procedures, and conditions representing typical process configurations.