In her next KLPA event, the Binhai Open, she became one of a few players to win three consecutive tournaments, and won by a comfortable margin of five shots. It was a great run and one that moved her into second on the KLPGA money list. Seo then won the Interburgos Masters later in the year for her fourth win of 2008. Seo also notched a third place finish at the S-OIL, and then won her amazing fifth KLPGA event of the year. The tournament was the Saint Four Ladies Masters, the first KLPGA event co-sanctioned by the Ladies European Tour. This victory gained her membership on the European tour, and entry into their elite field events like the Evian Masters. It also made her only the third golfer in KLPGA history to win five times in a single season on tour. And Seo still wasn't done: she finished the season by winning her sixth title of 2008 at the ADT-CAPS, the year's final tournament.
Seo continued in great form when the 2009 season started. She won the Lotte Women's Open in April, and followed that up with a brilliant win at the Korean Women's Open in May. It was her first Major triumph, but it would not be her last. She also finished second at the next Major on tour, the KLPGA Championship. She also won the Hite Cup Championship, the year's third Major, then won an epic battle with amateur Hana Jang to grab the year's fourth Major as well. In four Majors in 2009, Seo had three wins and a second, a truly fantastic achievement. For good measure, she also won the year's final tournament, the ADT-CAPS. Thus, she had five wins in 2009, and won the money list (with 663 million won earned), Player of the Year and scoring titles as well. Despite a great year from Ryu, Seo had done what she set out to do at the start of the year: establish herself as the KLPGA's best player.
Seo was awarded rookie of the year for the 2011 LPGA season. She carried on her good form in 2012 recording 8 top 10 finishes finishing runner up at the ISPS Handa Australian Open and the Manulife Financial LPGA Classic.