Ser-Od began participating in long-distance events in 2002. He made his international debut at the 2002 IAAF World Half Marathon Championships and stepped up a distance to the marathon at the 2003 World Championships in Athletics, where he came 63rd with a personal best of 2:26:39 hours. He was twelfth at the 2004 Hong Kong Marathon, the third Asian to finish in a largely African field. He made his Olympic debut that year at the 2004 Athens Olympics. He was just one of two Mongolians to compete in the athletics competition, alongside women's marathoner Luvsanlkhündegiin Otgonbayar. Ser-Od came 75th overall with a time of 2:33:24 hours, some twenty minutes behind the winner.