OCTA is responsible for funding and implementing transit and capital projects for the transportation system in the county, including freeway expansions, express lane management, bus and rail transit operation, and commuter rail funding and oversight.

Santa Ana Transit later merged with other, smaller agencies throughout the county, eventually leading to the formation of OCTD.

It has paid for the expansion on most freeways within Orange County, street improvements and repairs, traffic signal synchronization, and increased Metrolink service.

Some of the lines also extend to serve the Los Angeles County border communities of Artesia, Lakewood, La Mirada, Cerritos, Hawaiian Gardens and Long Beach.

These routes connect to Irvine Transportation Center and Tustin station, and have schedules timed to give passengers a smooth transfer to/from Metrolink and Amtrak trains.

The OCTA began funding rail operations with the Orange County Commuter, which Amtrak started operating in early 1990 running between Los Angeles Union Station and San Juan Capistrano with stops in Commerce, Fullerton, Anaheim, Orange, Santa Ana and Irvine.

With the Metrolink takeover in 1994, the southern terminus moved to Oceanside and five infill stations were added:[8] San Clemente and Norwalk/Santa Fe Springs in 1995,[9][10] Tustin and Laguna Niguel/Mission Viejo in 2002,[11][12] and Buena Park in 2007.

[13] Starting in 2005, OCTA has funded greatly expanded service on the Orange County Line, with trains now running twenty hours daily, seven days a week, as often as every 30 minutes.

Metrolink's Inland Empire–Orange County Line began operations in October 1995 between Riverside and Irvine; it was the first suburb-to-suburb commuter rail in the U.S. at the time.

[19] In February 2016, $125 million towards the project was included in the proposed United States federal budget for the 2016-17 fiscal year.

[21][22][23][24] The CenterLine was a proposed 9.3-mile (15 km) light rail system serving Irvine, Costa Mesa and Santa Ana.

Costing $1 billion, it was originally envisioned as a 30-mile (48 km) route that would run from Fullerton to Irvine, through Anaheim, Orange, Santa Ana and Costa Mesa.

OCTA street funding is steered towards roadways on the Master Plan in recognition of their role in regional travel.

Orange Freeway (SR-57): Work began in the summer of 2011 on the SR-57 to add a new northbound lane from Orangethorpe Avenue to Lambert.

OCTA purchased the 91 Express Lanes without taxpayer money and removed a "non-compete" clause that prevented safety improvements and traffic capacity expansions along the stretch of tollway.

In July 2003, OCTA adopted a toll policy for the 91 Express Lanes based on the concept of congestion management pricing, which is designed to optimize traffic flow at free-flow speeds.