(Currently there's no Terminal 3. The empty space between Terminals 2 & 4 is allocated for a future Terminal 3 in a master plan for future expansion.)
American Airlines (Gate 410), Southwest (Gates 401-407), US Airways (Gate 408).
LA/Ontario International Airport (IATA: ONT), , is the area's regional airport for commercial flights serving the Inland Empire in the eastern part of the greater Los Angeles area and adjacent deserts. It is less busier and quieter than LAX. The airport is located 2 mi (3.2 km) east of Ontario's Business District off of I-10 at the "Archibald Ave/Ontario Airport" (Exit #55/55A) and the "Haven Ave" (Exit #56) exits. Currently there are only two terminals for commercial flights numbered 2 & 4 with adjacent spaces allocated for a future Terminal 1 & 3. Major airlines serving Ontario are:
To travel from the airport to downtown Ontario (E Holt Blvd & N Euclid Ave (CA-83)) :
A taxi from the airport will cost around $16+ 15% tip.
There are frequent shuttle buses stopping at the outer islands in front of the arrival zones for transport to the car rental terminal at N Haven & E Airport Dr, Parking Lot #5 (NE of Terminal 4), and an inter-terminal shuttle between Terminals 1, 2 & 4 on three separate routes.
The 'on-airport' car rental companies are located in a separate consolidated terminal at 3450 E Airport Dr, in the NE corner of the airport grounds at E Airport Dr & N Haven. From Terminals 2 & 4 take a shuttle bus over to the consolidated car rental terminal to pick up a rental car. If renting from Advantage, Payless, Fox or another 'off airport' company they send their shuttle to pick up customers at the consolidated car rental terminal instead of the airline terminals. For a better rate on car rentals it is best to shop around and reserve a car online or over the phone PRIOR to arrival. Having a private car is the BEST way to get around locally in Ontario and the surrounding areas and for going out to remote locales such as Joshua Tree.
For public transportation the nearest stops to the airport are:
across the main from the front parking lot at N Archibald & E Guasti Rd served by #61 Goes west to the Pomona Transit Center via Ontario Business Distric (transfers to Amtrak, Metrolink and Foothills Trnasit in Pomona], goes north to the Fontana Metrolink Station via Ontario Mills Mall in the other direction.
The #81 & 82 go north & south along S Haven Ave, east of the airport property by the consolidated car rental terminal at E Airport Dr & S Haven Ave.
Local bus fares costs $1.50 for one ride or $4.00 for a day pass and takes 20 minutes to travel from Archibald Ave & E Guasti Rd
Despite the bad air quality, Ontario has a pleasant arid climate, with day summer/fall temperatures rainging from 92-102 and winter/spring night temperatures dipping into the upper 20s and low 30s. Winter and spring are the wettest times of the year, but even then, there still is little rain, and summer and fall are the driest, sunniest times of the year.
Ontario is located along Interstate 10 (I-10), west of the Interstate 10 & 15 junction, approximately 37 mi east of Los Angeles (39 min), 24 mi (25 min) west of San Bernardino, 71 mi (1 hr) west of Palm Springs,and 114 mi (2hr 14min) northeast of San Diego (off of Interstate 15).
From Los Angeles, simply take I-10 (San Bernardino Fwy) East (or west if coming from San Bernardino or Palm Springs). Ontario is accessed from exits #49 ("Central Ave") through #59 ("Etiwanda Ave") along I-10. Ontario does not have a clearly defined downtown except the closest thing would be the neighborhood NE of the intersection of N Euclid Ave & E Holt as that is where city hall and other city government function is located at. To get there, exit off at the N Euclid Ave (CA-83)/Upland off ramp at Exit 51 and go south along N Euclid Ave To E Holt. Likewise one can also go east on CA-Hwy 60 which goes along the south side of town. From CA-Hwy 60 (Pomona Fwy) Ontario is accessed from Exits #34 ("Mountain Ave") through Exit #41 ("Milliken Ave")
From San Diego take I-15 North until CA-HWY 60 at exit 106 or the next one up at I-10 at Exit #109. Go west on CA-60 or I-10 to get to Ontario.
Amtrak, 198 E Emporia St. The Sunset Limited runs three times a week out to New Orleans via Tucson and San Antonio. Additionally, Amtrak's Texas Eagle service between San Antonio and Chicago incorporates the Sunset Limited to provide a direct connection to the Los Angeles Union Station. The Sunset Limited/Texas Eagle stop in Los Angeles Union Station, Pomona, Ontario and Palm Springs in California.edit
The Metrolink commuter train  also offers service from LA's Union station to Ontario or near Ontario on the San Bernardino and the Riverside lines. From the San Bernardino Line the nearest stations are 'Montclair', 'Upland' and 'Rancho Cucamonga'. From the Riverside Line the nearest stations are 'Pomona' and 'E Ontario'. Each station is served by Omnitrans bus (to Rancho Cucamonga, Fontana, Ontario, San Bernardino, Upland, etc) and/or by Foothills Transit (Pomona, Claremont, Montclair, West Covina, Chino, La Verne, etc) for onward travel.
The best way to get around Ontario is by car. Car rental agencies are located at or around the airport. Otherwise Omnitrans operate buses in and around Ontario, Fontana, Rancho Cucamonga, San Bernardino, Upland, Yucaipa and the surrounding areas in San Bernardino County.
Ontario Mills Mall, 1 Mills Circle (SE of the Milliken Ave & E 4th St intersection. Accessed from I-15 at E 4th St @ Exit #110 and from I-10 at N Milliken Ave @ Exit #57), . Mon-Sat 10AM-9PM, Sun 10AM-8PM. One of the largest single-story malls in the West Coast with an abundant number of stores within the mall and in the surrounding area.edit
A number of hotels nearby the airport also offer shuttle services (for free or for a fare) from the airline terminals to their respective properties. Some may contract with the same shuttle company as their competitor(s) so the same van or bus may make several stops at different hotels to pick up/drop off guests. It is recommended to arrange for pick-up 72hrs prior to arrival. 
Ayres Hotel and Suites - Ontario Convention Center, 1945 E. Holt Blvd., Ontario, CA 91761, ☎ 909-390-7778, . checkin: 3pm; checkout: 12pm. Ontario's Only Full Service Upscale Boutique Hotel - Offering complimentary full, hot breakfast; parking; wireless internet & shuttle service. In-room microwave and refrigerator. edit
Best Western Country Inn, 2359 S Grove Avenue, ☎ +1 909 923-1887 (toll free: +1 800 770-1887, fax: +1 909 923-4504), . edit
Best Western Innsuites Hotel & Suites Ontario, 3400 Shelby Street, ☎ +1 909 466-9600 (toll free: +1 800 642-2617, fax: +1 909 941-1445), . edit
Best Western Ontario Airport, 209 N Vineyard Avenue, ☎ +1 909 937-6800 (fax: +1 909 937-6815), . edit
Ramada Ontario Airport Hotel, 1841 East G Street, Ontario, CA 91764, ☎ 909-988-0602, . Our Ontario, California hotel offers a convenient location, free wireless internet, complimentary breakfast, outdoor swimming pool and Jacuzzi suites.edit
Red Roof Inn Ontario Airport, 1818 E. Holt Blvd (two blocks from Ontario Convention Center, at the entrance to the Ontario Airport), ☎ 909-988-8466, . checkin: 2PM; checkout: 11AM. edit
Holiday Inn Ontario (Ontario, CA Hotel), 2155 East Convention Center Way Ontario, California 91764, . edit
Ramada Ontario Airport, 1841 East G Street, ☎ 909-988-0602, . Situated east of Los Angeles, Ontario, California was once a town abounding with agricultural communities and never ending citrus fields.edit