Recent developments in the manufacture of steel require the use of large quantities of high purity oxygen. To supply this demand new air separation plants have had to be designed and built which are capable of producing oxygen of large tonnage quantities at a minimum purity of 99.5%. This paper will describe in detail the 100 ton/day oxygen units built for Dominion Foundries and Steel Company, at Hamilton, two of which are now in operation. The process flowsheet and the reasons for adopting it is explained. Some of the problems encountered in the choice of certain specialized pieces of equipment are considered. Details of flow, pressure and temperatures are presented for normal operating conditions as well as operating control, methods, and measurement. The paper is illustrated with pictures of the equipment and the assembled plant.