Conn, who announced last July that he planned to quit, had been under pressure from shareholders because of the company's poor performance. Trade unions had also criticised his 44% pay rise to 2.4 million pounds in 2018 amid a restructuring that included thousands of job losses.
O'Shea, Conn's immediate replacement, was appointed Chief Financial Officer in 2018, and will continue as interim CEO until a permanent one is identified.
The company also said Chairman Charles Berry will be replaced by Scott Wheway, with Berry's decision to leave coming after advice from doctors to reduce his workload.
Separately, engineering company Weir Group said it expected Berry, who is also its chairman, to return within the next few weeks.
(Reporting by Muvija M in Bengaluru; Editing by Alex Richardson)
