I traded in my bag of Olympus junk last year and picked up a Nikon D700 with a Sigma 120-400 4.5-5.6 lens.
I have found over the years that shooting aperture priority at F8 is your best bet for a sharp picture.
The beauty of the D700 is that increased iso does not mean increased noise so when you need the extra speed, just increase the iso - even 800 plus still gives excellent results.
The Sigma lens is just as good as the canon or nikon offerings, and at least half the price.
The main thing to remember is to shoot at at least F8 if you can, it's the lenses sweet spot and will give you the best results .
Shooting wide open will not unless you pay 4k or over for a superfast lens.
And IS rocks , in any situation !