The 30-year-old Madeleine is the youngest of King Carl XVI Gustaf and Queen Silvia’s three children and is fourth in line to the throne.
The tabloid Expressen reported on its website early Thursday that O’Neill proposed to Madeleine in New York about a month ago after asking the king for his daughter’s hand in marriage.
Officials at the royal palace didn’t immediately comment on the report, but a guard answering the phone at the palace said “information will be available” on the palace website later Thursday.
Madeleine lives in New York where she works for the World Childhood Foundation, a nonprofit founded by her mother.
In 2010, Madeleine broke off an earlier engagement after media reports that her then-fiance had cheated on her.