McCarthy, who was not charged with any crime, told officers he was "doing well and wanted to bless others with gifts of money," the OSP said in a statement to KVAL-TV.
The money is believed to have come from McCarthy's family's shared bank account.
Police warned McCarthy he could be cited for littering or creating a hazard on the highway and he agreed to stop throwing the cash.
OSP Capt. Kyle Kennedy said other drivers stopped on the highway and quickly collected the $100 bills.
"OSP is urging the public not [to] go searching for money on the highway -- which is very dangerous and contributes to a hazardous condition," Kennedy told OregonLive in an email.
Kennedy said officers searched the highway for any leftover cash, but didn't find any.