Oropeza was successfully treated for liver cancer in 2004, but fell ill again earlier in 2010. She died of complications from a blood clot in her abdomen.
In light of the late senator's victory, the secretary of state's office said a special election will be called to fill Oropeza's vacancy.
In county elections, John Noguez won the race for Los Angeles Assessor. Noguez took 820,947 votes, amounting to 59.8 percent, and his challenger John Wong won 551,966 votes, amounting to 40.2 percent. Noguez is a 25-year veteran of the assessor's office.
Also, a democrat won an open state Assembly seat in the South Bay. Betsy Butler beat out Republican Nathan Mintz of Redondo Beach in the 53rd Assembly District. Butler wound up with 50.3 percent of the vote.
In an election ruled by the GOP, Democrats maintained a stronghold of many Los Angeles County-area U.S. congressional districts. These districts cover Burbank, Glendale, Los Angeles, Santa Monica, Culver City and Long Beach.
Click here for a complete list of local election results.
The Associated Press contributed to this report.