Dpreview's review of the OM-D said that while the OM-D had a measurably higher noise level at higher ISOs, the difference between it and the better APS-C cameras was so minor at to be unimportant for normal-sized viewing and prints. To get a truly significant difference, you really needed to move up to full-frame, with attendent size and weight gain.
It really depends on what your priorities are. If you print really big (bigger than 11x14), the K5 will probably be a little better. If you want real portability, a camera you will be more likely to take everywhere, the OM-D wins. Both are excellent cameras with excellent lenses. If you really are concerned about the least noise at the pixel-peeping level at very high ISO (6400 and up), the K5 wins, but the D700 or D800 are even better, and by a much larger margin.
Both systems (K and OM-D) have excellent lenses, so the question there is which one has the specific focal lengths and speeds you want. Be aware that the K5 has a stronger anti-aliasing filter, I think that OM-D pictures are a little sharper.
Another question is whether you shoot JPG or RAW. If JPG, then the question is how good each camera's internal NR is. Does it reduce the noise to a tolerable level without sacrificing important detail? If you use a RAW converter, then the question is whether that converter handles one system's noise pattern better than the other.
These cameras are systems, along with the RAW converter (if any) that you use. It's less useful to consider one parameter (noise) in a vacuum, more useful to view them as a whole.
Full disclosure: I considered and tried the K5 (and the D7000), but just bought an OM-D, so I'm in the "honeymoon" period with it. I also own a Leica M8.