[3] After getting a degree in law,[4] Barry O'Callaghan got a job at investment bank Morgan Stanley, where he "quickly prospered"[3] working in mergers & acquisitions.

[3][5] In 1997 O'Callaghan got a senior job the telecommunications and technology division of investment bank Credit Suisse First Boston.

[6] O'Callaghan quit Credit Suisse First Boston in 1999 to become the Chief Executive Officer of the digital publishing startup, Riverdeep,[3][5] in order to take the company public.

[2] Just as CD-ROMs were gaining popularity, O'Callaghan transitioned Riverdeep away from CDs into offering online subscriptions to educational content.

[6] He led the acquisition of a software company called Logal, which sold math curriculum online.

[2] From 2003 to 2004, he "streamlined" the company, negotiated 300 million euro in bonds and worked to expand into international markets, like the United Kingdom and Saudi Arabia.

[12] The Houghton Mifflin brand also fell out of favor with many in the literary community after discontinuing some of its contracts with writers.

[3] O'Callaghan became Chief Executive Officer of Houghton Mifflin Harcourt in April 2009, after the prior CEO retired.

[3] In 2010, bondholders that Riverdeep was indebted to converted their debts into equity in the company,[17] and took control of the business[12][17] in order to avoid bankruptcy.

[12][17] O'Callaghan said the company's financial problems were the result of unpredictable market circumstances for textbook purchases and paying too much for the Houghton Mifflin business.

"[3] The Irish Times said O'Callaghan responded to the crisis quickly thanks to "deft management" and the "loyalty of his business partners."

Business author Paul Carro said O'Callaghan had built "a house of cards" where failure was inevitable.

[18][20] He personally owns exclusive rights to some of HMH’s educational materials for teaching the English language.