Oceanside is about 40 miles from the San Diego International Airport (SAN) and 50 miles form the John Wayne airport in Orange County (about 45-50 minute car ride). There are more direct flights from far-away places into San Diego.
Getting Here by Train
Some people have expressed an interest in traveling to the wedding hotel by train rather than by car. That is relatively easy because the SpringHill Suites is next to the Amtrak/Metrolink station in Oceanside (OSD).
Both Amtrak and a regional service called "Coaster" have regular trains that run between San Diego's Santa Fe Depot (SAN) (which is downtown, about a $7 Uber ride from the San Diego airport) and Oceanside Depot (OSD). The trip takes about an hour.
Tickets for Coaster Trains are purchased at a kiosk and can be as cheap as $5 one-way. The Coaster website is here, including a schedule: http://m.gonctd.com/coaster/
From Orange County, the Metrolink regional rail service runs at least a half dozen trains per day to Oceanside. The Tustin Station is closest to the airport and less than a $10 Uber ride. The Santa Ana or Irvine Stations are also near. Seehttps://www.metrolinktrains.com/