O’Neill, who was head of Australian oil and gas firm Woodside Energy, will be the the first woman to run a major global oil firm.
Carol Howle, BP’s current interim chief executive, said the company looked forward to O’Neill’s arrival “as we accelerate our progress to build a simpler, stronger and more valuable BP for the future”.
The company has come under pressure from its shareholders for underperforming compared to its rivals in recent years.
A year ago, BP announced a change in strategy as it cut planned investments in renewable energy projects and said it would spend billions of dollars more a year to refocus on its core oil and gas operations.
The energy giant is trying to cut its debts, which currently stand at about $22bn.
Announcing its latest results, BP said it aimed to make cost savings of $5.5bn-$6.5bn by the end of 2027, which is an increase from its previous target of up to $5bn.
Profits in the final three months of the year fell 30% to $1.54bn, in a period when the price of Brent crude oil fell below $60 a barrel for the first time in more than four years.
Rival oil giant Shell also announced a fall in profits when it posted its annual results last week. Shell reported underlying earnings of $18.53bn for 2025, a 22% fall on the previous year.
O’Neill takes over at BP at an uncertain time for the oil giant. Its previous boss, Murray Auchincloss, stepped down after less than two years in the job.
He had replaced Bernard Looney, who was dismissed by BP in 2023 after he was found to have committed “serious misconduct” in failing to disclose relationships with colleagues.