* Article updated 21 of Jan, 2024 *
This article is not sponsored by Olympus in any way. What I write here is my experience after taking thousands of images with an Em-10, Em5 Mark II, Pen-F and a EM1-X over a span of 9+ years. Most of the images in Theory4 have been shot with these cameras for professional and personal use.
There is an old misconception that many photographers still have today about cameras: the bigger the sensor, the better the image quality. Coming from a Nikon D800, a D700, D300, D200, and having used a couple of Fuji X cameras, I was just as skeptical as everyone else the day I won an Olympus Em-10 in a local photo contest. I decided to have a laugh and took the E-M10 for a spin - kit lens and all - the next day.
At base ISO, the E-M10 raw's looked just as good as my old champion, the Nikon D800. White balance was always SPOT ON (as opposed to Nikon's warm colour casted images back then) and tone & sharpness seemed better. What was supposed to be an entry-level toy of a camera soon to be designated as door-stop had just stood toe to toe with one of the best cameras I ever owned.
But there's more.
The raw files had enough latitude to recover plenty of highlights, and the noise was not as bad as everyone claimed. The other complaint which I kept reading about was the system's inability to produce bokeh easily because of it's 2x focal length multiplier, but this was also disproved once I got my hands on the 17mm f1.8 and the ridiculously cheap 45mm 1.8 lens. So what was going on here!? Why wasn't everyone jumping ship to this compact bad-ass system that included image stabilisation, 1/250 flash sync speed, and had lenses half the size and weight of full frame cameras?
Because no professional took it seriously. Few Pro's are willing to take the time to learn these cameras and use them for professional work out of a misconception that it won't deliver.
In 2015 I gave this system a proper shot. The following notes are the result of hundreds of hours of experimentation with Olympus cameras both in the field and in post. I abandoned my Nikon gear and worked exclusively with this new system for personal and commissioned assignments. Here are my findings.
01 / Set Adobe Color as the calibration profile in Lightroom
Forget the other colour profiles (camera natural, camera neutral, portrait, etc). Adobe Color is the way to go if you want maximum dynamic range and colour accuracy. This goes in contradiction to the popular trend of selecting a profile closest to the in-camera setting, which delivers punchy Olympus colours at the expense of dynamic range and out of gamut reds. If you find your images too flat after switching to Adobe Standard, simply add +45 to the blue primary saturation slider in the camera calibration box.
02 / Calibrate your camera with a colour checker
To get accurate colours in Lightroom & Photoshop you need to calibrate your camera with a colour reference target. I use the SpyderCheckr from Datacolor, but any will do. This is critical for displaying realistic colours in LR & PS. The following example compares the same image of a color checker target without and with the corrections applied: 
Without camera calibration.
With camera calibration activated.
Notice the hue shift in the blues. The calibrated image shows a true blue sky, while the uncalibrated image shows cyan skies. The difference in these images is subtle, yet it becomes more apparent when you start pushing the raw files. White balance was set to auto.
03 / Pre-processing with DXO Pure RAW
The launch of DXO Pure RAW 2 changed the game for MFT cameras in a big way. This pre-processing software converts the Olympus  (now OM-System) ORF files into DNG's  with a number of targeted corrections, the most important of these being DEEP PRIME noise reduction and  far superior sharpening.  Now ORF files become essentially noise-less at all ISO values, making the files feel more full-frame then ever.  Noise on MFT systems has always been a problem, even at base ISO, but no more thanks to Pure RAW.  Fuji's RAF files also benefit in a big way from being passed through this software, since Lightroom does not de-mosaic the shadows very well on cameras with the X-TRANS sensor.
Standard EM1-X ORF file, ISO 200.
Passed through DXO Pure Raw 2.
100% View
100% View
300% View
300% View
Mean value: 144,69
Mean value: 148,90
Notice also that the average brightness of the image - the Mean value - has gone up by almost 5 points due to the optical corrections applied by DXO. The resulting image, not touched in in anyway by Lightroom, is sharper, brighter, and completely free of noise.