Now I really feel like a slacker. As far as I can tell, Oscar is not using a C-leg but a mechanical knee 3R80 (I had one of those previously and I never could figure out how to run with it). Check out the video on the link:
http://www.stuff.co.nz/auckland/local-news/east-bays-courier/5308307/Nothing-is-impossible
running is normally done with a mechanical leg. the C-leg is set up for walking or other activities you might be able to have the 2nd mode set up to run I use mine to dance so I do not fall down some how it runes the guys immage if he dropps me....
I do have a C-Leg now and I find it easier to run or walk down the stairs than with the 3R80. I fell many times with the 3R80. With the mechanical leg, I found you have to be more mindful on the position of the knee (should be fully extended) when the prosthetic foot hits the ground otherwise the knee will quickly bend and when you go down the stairs, you have to make sure you put all the weight on the prosthetic foot before trying to bend the knee otherwise ouch! I think Oscar mastered the technique and I didn’t 
The C-leg is a blessing though…
I also read about this one! A great inspiration to all our members! 
Thanks for sharing!