Roberts notified the Board of Regents in a letter Friday his resignation is effective immediately. The board is to meet next week to decide how to go about replacing Roberts, the university said on its Web site.
In October, Roberts took a leave of absence from the school founded by and named for his 89-year-old evangelist father, after three former faculty members filed suit against ORU, Roberts and three university administrators -- accusing the school of paying for horses, hot tubs and plane trips for Roberts and his family.
The suit alleges Roberts' family used ORU funds to pay for expenses including housing, remodeling of dorm rooms for Roberts' daughters, $51,206 worth of clothes and phone bills.
Roberts has said he pays for his family's expenses.
The plaintiffs allege they were fired after they reported information indicating that Roberts and his family spent school funds on themselves.
The lawsuit has been amended to include accusations that the school gave "a convicted sexual deviant unrestricted access to students" and shredded evidence after the suit was initially filed, CNN reported.