With blending Ha and RGB data, As Doug said, it can be really difficult to get rich colours, its either too saturated, or too pastel most of the time..
I was poking around on the net looking for tutorials on the best ways to do it, and found
THIS on Rob Gendlers site... If you blend your ha 60~80% into your red channel, and then construct your RGB image as normal, you'll find that the colours come out a hell of a lot nicer. You'll get the rich colours you're used to getting with RGB data or OSC data whilst maintaining the extension and detail given by the Ha data...
Its a really good read, very easy to follow and I've found since doing this that my Ha:OSC shots have improved greatly.