Omwanza made a number of podium finishes in major races in 2002: she had runner-up placings at the Paris Half Marathon, Humarathon and Lille Half Marathon, and managed third at the Nice Half Marathon and Semi-Marathon Marvejols-Mende. She made her debut over the full marathon distance that year and came second to Lucy Karimi at the Reims Marathon, finishing the 42.195 km distance in a time of 2:35:36 hours. A year later, in only her second outing in the event, she improved her best by nearly eight minutes to win the Paris Marathon in 2:27:44 hours, defeating Rosaria Console in a sprint finish. She was selected to represent Kenya in the women's marathon at the 2003 World Championships in Athletics, but she did not manage to finish the race as she dropped out after 25 km. Omwanza went on to win the Reims Marathon in October that year, although her winning time of 2:32:02 was somewhat slower than her Paris run.