The biggest achievement of her career is a win at the Paris Marathon in 2003. She has also won the Turin Marathon and Reims Marathon. Over the half marathon distance, she has competed extensively in France and has won at the Paderborner Osterlauf, Great Scottish Run, and Bredase Singelloop.
The Kenyan made four appearances in the marathon in 2004. Her first race at the Los Angeles Marathon saw her off form, as she finished eighth, but she improved to take third place at the Ruhrmarathon in April. Omwanza achieved a career best time for the distance at the Berlin Marathon in September: she crossed the line in 2:27:19 hours for fourth place, although the winner Yoko Shibui was far ahead of her as she became the fifth woman under 2:20:00 hours.[13] She ended the year with a win at the Great Scottish Run in Glasgow and a fifth place at the Nairobi Marathon.[6][14]
She ran two marathons in 2006, taking third place on the podium at both the Nairobi and Madrid Marathon. A season's best in the half marathon came at the Berlin race, where her time of 1:11.35 was enough for fourth. She was third at the Marrakech Marathon and second at the Paderborner competition that year.[6] In 2007, she claimed third place at the Hamburg Marathon with a run of 2:30:46 hours and also won the Paderborner Half Marathon and Nacht von Borgholzhausen 5-mile races.[15][18] Her last season of competition was 2008: she had podium finishes at the Vienna City Marathon and Marathon De La Rochelle in France.[6] She also ran at the Berlin Half Marathon, coming sixth,[19] and was third at the 20 van Alphen. In October, she won the Bredase Singelloop.[20]