OST, which was spun out of BHP in October 2000, markets products used in the construction, manufacturing, housing, mining and agricultural industries.
OneSteel announced a 1H FY12 net loss of $74 million, swinging from a $116 million profit a year earlier.
The results were marred by a $130 million writedown on the groups LiteSteel business in the Australia and the US.
The group said that it expects sales of approximately two million tonnes in 2013, after its Peculiar Knob project begins operations in the December quarter of this year.
The company declared an interim dividend of $0.03 a share, unfranked.