What can you do to illustrate O-rings? Just draw two concentric ellipses and fill them black? This cannot be all. It's the details that count...

You often see O-rings in technical illustrations that look as if they are far beyond the maximum of their life-cycle: flat and expressionless. One reason for this can be that an ellipse is drawn and the same ellipse with the same angle is put around it. This is the right approach, but one detail is missing: the ellipse angle of the bigger ellipse must be bigger than the one of the smaller ellipse. Only then the element looks like a ring with a round diameter. Otherwise it looks flat and expressionless.