"It is in the domestic politics and economic policy realm where the ultimate success of Haitham's leadership will be determined," said Robert Mogielnicki of the Washington-based Arab Gulf States Institute.
Rated junk by all three major agencies, Oman's debt to GDP ratio spiked to nearly 60% last year from around 15% in 2015, and could reach 70% by 2022, according to S&P Global Ratings.
It faces rising refinancing risks as a result of large government external debt maturities in 2021 ($4.3 billion) and 2022 ($6.4 billion) that could add significant pressure to foreign exchange reserves if the debt is not rolled over, S&P associate director Zahabia Saleem Gupta said.
The relatively small oil producer has relied heavily on debt to offset a widening deficit caused by lower crude prices and held back on austerity measures that could cause unrest.
"The country is pursuing a sustainable fiscal adjustment path: austerity measures are to be combined with a decisive impulse towards diversification," said Fabio Scacciavillani, chief economist at Nuverse and former chief economist at an Omani sovereign wealth fund.
"It is a delicate balancing act which might encounter resistance in certain quarters," he said.
Qaboos was finance minister and central bank chairman. He was also premier, defence and foreign minister and head of the armed forces.
He resisted turning to Gulf neighbours for financial aid in order to maintain an independent foreign policy.
When protests broke out in Oman during the 2011 Arab Spring, Qaboos sacked more than a third of the cabinet, created thousands of jobs and gave money to the unemployed.
Haitham cannot afford such largesse.
"Qaboos had too many roles and was reclusive and that meant his advisers would not tell him the challenges the country was facing," said a diplomat in Oman.
"Inevitably he (Haitham) will be more receptive to the challenges of the country."
Haitham, who was culture minister and in charge of Oman's development plan, is likely to form a team of policy advisers after 40 days of mourning, which started on Sunday.
If he appoints a prime minister and moves to decentralize power this would signal willingness to improve decision making, the diplomat said.
Oman will be the Gulf's worst performing economy over the next two years, with gross domestic product growth forecast at 0.5% this year and 0.8% in 2021, according to Jason Tuvey, senior emerging markets economist at Capital Economics.
This year, it plans to raise over $5 billion in debt to partly cover some $6.5 billion of estimated deficit, equivalent to 8% of GDP.
It will cover the rest of the deficit by drawing from reserves, already eroded in recent years at a pace which has raised concerns over the sustainability of the Omani rial's peg to the U.S. dollar.
Fiscal slippage could occur in Haitham's first year, when spending tends to increase, said Carla Slim, economist at Standard Chartered.
But his ascension "could act as a catalyst for the pace of reform and medium-term fiscal planning", she said.
Oman has delayed bringing in a 5% value added tax from 2019 to 2021, and economic diversification has lagged, with oil and gas still accounting for over 70% of government revenues.
(Additional reporting by Marc Jones; Editing by Giles Elgood) ((Davide.Barbuscia@thomsonreuters.com; +971522604297; Reuters Messaging: firstname.lastname@example.org))