O'Brien has spent most of his career in the communications technology and mass media industries. He has also been a part owner of energy, aerospace, and industrial service companies.
O'Brien was until early 2021 the owner of Communicorp, a media holding company operating across Europe. He started the company in Ireland in 1989, where it has owned independent radio stations like Newstalk and Today FM. The company expanded to markets in Eastern European countries, later selling some of its stations to local operators. In 2014, Communicorp expanded to the United Kingdom, acquiring eight radio stations across the country; in 2017, Communicorp moved its UK radio stations to a new independent company, Communicorp UK, of which O'Brien owns 98% of the shares. In February 2021, O'Brien sold Communicorp to Bauer Media Audio for approximately €100 million.
In 1991, O'Brien formed a telecommunications consortium called Esat Telecom to compete with the state-owned Telecom Eireann. In partnership with Telenor, Norway's state telecom operator, Esat formed Esat Digifone, which made a successful bid for Ireland's second GSM mobile licence. Circumstances around the awarding of the licence to Esat Digifone became the subject of the Moriarty Tribunal.
On 7 November 1997, Esat Telecom Group plc held an initial public offering and was listed on the Irish Stock Exchange, London Stock Exchange, and NASDAQ. In 2000, Telenor made a bid for control of the company, but O'Brien sold it to BT, reportedly making €250 million from the sale.
In 1999, O'Brien co-founded aircraft leasing company Aergo Capital, in which he owned an 80% stake. From its inception until 2014, Aergo traded more than 150 aircraft with a gross value of over €791 million (approximately $1 billion). In October 2014, O'Brien and his partner, Fred Browne, sold the company to CarVal, a US investment firm; Browne remained with the new company as CEO.
In the late 2000s, O'Brien began purchasing shares of Independent News & Media (INM), ultimately spending an estimated €500 million to amass a 29.9% stake in the company. O'Brien clashed with the company's board, especially former owner Tony O'Reilly, who stepped down from his position as CEO in 2009 and sold most of his INM shares in 2014. In April 2019, O'Brien and Dermont Desmond, INM's second largest shareholder at the time, sold their shares to the Belgian media group Mediahuis; O'Brien reportedly received €43.5m as part of the deal.
Although he never owned a majority stake in INM, O'Brien was at times accused of exerting significant influence at the company. In 2014, allegations arose that Stephen Rae, a group editor at INM, ordered amendments to a column by Sunday Independent editor Anne Harris that contained references to O'Brien. In 2015, Paul Meagher, a solicitor for O'Brien, reportedly called INM solicitor Simon McAleese in 2012 to block a story related to environment minister Phil Hogan.
In March 2018, the Office of the Director of Corporate Enforcement (ODCE) in Ireland applied to the High Court of Ireland to appoint inspectors to Independent News and Media to investigate an alleged data breach. According to an affidavit filed by the ODCE, invoices for the data interrogation were discharged by Blaydon Limited.
In June 2019, Independent News & Media was sold to the Belgian group Mediahuis and de-listed from the Euronext Dublin exchange, thus ending O'Brien's involvement in the group. It was reported that he lost in excess of €450m in total on his INM investment.
In 2001, O'Brien founded Digicel, a telecom company that operates in the Caribbean, Central America, and Asia Pacific. Using the cash from his sale of Esat Telecom, O'Brien used Digicel to build a wireless network in Jamaica. That same year, Digicel expanded into the South Pacific. As of 2022, Digicel operates in 25 countries.
Along with Digicel, O'Brien created the Digicel Foundation, which has worked with local organizations to develop community services, build schools and health centers, and support recovery efforts. After the 2010 Haiti earthquake, O'Brien pledged €3.5 million to assist recovery efforts. In 2012, President Michel Martelly of Haiti awarded O'Brien with the National Order of Honour and Merit for his investments, contributions and promotion of the country, and in 2015, O'Brien received honorary membership of the Order of Jamaica for his service to the country's telecommunications industry.
In May 2020, Digicel filed for bankruptcy court reorganization and successfully sought a debt restructuring agreement. A debt exchange was approved by the company's bondholders and took place in June 2020. As part of the exchange, O'Brien agreed to contribute $50 million of assets to Digicel, including $25 million in cash and the company's Jamaican headquarters.
In October 2021, Digicel announced the sale of its Pacific operations to Telstra Corp, an Australian telecommunications company. Valued at US$1.85 billion, the sale was completed in July 2022, with Telstra agreeing to pay Digicel US$1.6 billion before adjustments and deductions. Telstra purchased all six Digicel Pacific markets, including Fiji, Nauru, Papua New Guinea, Samoa, Tonga, and Vanuatu.
In 2012, O'Brien purchased Siteserv, a utilities support company, from IBRC for €45m; in 2015, the company was renamed Actavo. Actavo was bought and controlled through O'Brien's firm Millington on the Isle of Man.
In 2016, Actavo expanded into the United States through the purchase of Atlantic Engineering Services, a structural engineering firm. Actavo was also involved in the installation of fibre networks for Digicel in the Caribbean.
In December 2013, O'Brien purchased €300 million in debt owed by Topaz Energy to the Irish Bank Resolution Corporation. In December 2014, Topaz's parent company, Kendrick Investments, announced it would buy all of Esso's Irish operations.
In December 2015, Alimentation Couche-Tard, a Canadian convenience store company, announced that it planned to buy Topaz. The sale was completed in February 2016; Topaz had more than 2,000 employees and close to 35% of the consumer market in Ireland at the time of the sale.
O'Brien has attended the World Economic Forum's annual winter meeting in Davos, Switzerland, alongside other billionaires like Bill Gates and George Soros.
In 2000, O'Brien became a director of the Bank of Ireland, and in September 2005, he was named a deputy governor of the bank. In September 2006, he resigned as both deputy governor and as a member of the bank's court (board). O'Brien reportedly resigned due to increased demands related to his international business interests.
O'Brien considered purchasing Doncaster Rovers in the early 2000's, having owned the club's ground Belle Vue since 1998.
Between 2008 and 2016, O'Brien gave the Football Association of Ireland (FAI) as much as €12 million to help pay the salaries of senior officials within the organisation, including Giovanni Trapattoni. In 2018, O'Brien was named Honorary Life President of the FAI.
In 2006, O'Brien purchased a 2.82% stake in Glasgow-based Celtic F.C. from a former manager of the club, Martin O'Neill. As of June 2018, O'Brien reportedly had increased his ownership to 13%.
In 1998, O'Brien purchased Planal SA, the holding company for the Quinta do Lago golf resort in Algarve, Portugal.
