Disclaimer: I don't own either of these cameras (being a starving artist who makes do with older models) but I have used the K-5 and (briefly) E-M5. And I have used both of these camera systems... extensively.
First, I don't think anyone has addressed the DNG issue in the OP. I love that Pentax shoots natively in that format. For Olympus I simply use the free Adobe DNG converter. With Lightroom you don't
need to, but I prefer that workflow. Problem solved.
People argue back and forth which is the best camera for auto-focus. I think both of these will do fine for relatively immobile subjects. For tracking get a Canikon.
As for physical heft, just try to
see the difference. You know, in prints. Sometimes I wonder if people here actually take photos or just argue about numbers. Yeah, I know, it's "the internets".
How large do you need your prints to be? At what resolution? Maybe you need a full-frame or medium format camera instead of these little "cropped sensors".
As for size, I much prefer Olympus, which is why I (largely) switched over. I can pack a high quality prime kit in a bag no-one even notices -- the same space might just fit a Pentax body. That is a real win for me. People complain about cameras being "too small" and then use a smart phone with tiny controls and itsy screens. I don't get it. I have never found a camera that as too small, but I have found some badly designed ones. Then they might feel small, or, more precisely, simply wrong. But I like the usability of Olympus and Pentax (Fuji too).
But if you need a big camera to impress the chicks (or the dudes) then, heck, just buy a Canon.
For lenses, it's best to first
ask what you need, and then see if the system can deliver. Pentax still doesn't have a fast normal prime, something we've been banging on about for ages. It does have some superlative glass in the FA Limited series. Few brands touch that. But they haven't advanced on those old designs.
But MFT has some amazing lenses as well. Right now I carry the Panasonic 14mm and Olympus 45mm. Images are every bit as good as most Pentax glass (reserving only the FA43 / 77 for special consideration). And now there is the 75mm with stellar rendering that seems to be tackling the FA77 head on.
Of course on the MFT system you can use any legacy lens ever made, so long as you don't care about AF. That is a massive advantage over Pentax, even if there are a good number of M42 and K-mount lenses floating around.
(Oh yeah, I should mention that, as a photographer, I know where I want to focus. I don't want to bet on a computer doing it for me. YMMV.)
I can't tell you about flash, since I use exactly the same trigger to exactly the same pots (Metz) on either camera.
The EVF system on the Olympus is far superior to Pentax. Having all my controls in a HUD gives me more control without removing my eye from the finder. No need for top panel on the camera; no need to look at the back LCD. Plus I can magnify in on the focus point, ramp up the light to see in the dark, etc. I was sceptical and am optical viewfinder aficionado until the EVF quality got good enough to convince me otherwise.
Pentax is a more responsive camera, has slightly higher dynamic range, and seems better built to me... though things have been slightly downhill since the K20D. The system has more weather-sealed lenses, though if you are using primes I wouldn't worry so long as the body is sealed. I mean heck, I took the DA12-24 right under Niagara Falls and was simply
careful. Cameras can't do your thinking for you.