Running Outlook on the iMac causes the screen to flicker and eventually black out and reduce to half/split screen display.

The only think I would say with this is that I never had a problem before upgrading to Mojave. When I previous OS versions like Sierra and El Captain I never had the issues. So Apple Can say it's Microsoft but maybe it's the Apple OS. I've deleted and reinstalled the latest version of Outlook on the 18/06/2019 and it does the same thing!

Aplle has got nothing to do with this. I've had this issue way back before Mojave and dark mode was released. Actually i was hoping Mojawe would fix this issue, but it did not.

Meanwhile i fixed the issue by buying an iMac 2019. Thanks Microsoft....

I also have a 2015 27" iMAC and am having this same issue with Outlook 365.

I believe it to be a video driver issue and of course, you can't download or update MAC AMD video drivers.  I think some video call is being made by Outlook that the driver doesn't like.

As far as I can tell only machines with the M9 series (possibly specifically the 390) series is affected.

Does anyone have a different video card than that?

I have tried light mode in both Outlook and macOS but that did not help me.

I have tried reinstalling O365 and that did not help me.

Apple recommended an OS wipe and reload but that is not going to solve this, they just don't know where to go next.

I have contacted AMD with the relevant details and I will post any replies I get.  There are rarely video driver updates from Apple/AMD where people are used to almost monthly releases from NVidia with new features and fixes.

Looks like you are right.   Details of my graphic card below:

AMD Radeon R9 M395:

  Chipset Model: AMD Radeon R9 M395

  Type: GPU

  Bus: PCIe

  PCIe Lane Width: x16

  VRAM (Total): 2 GB

  Vendor: AMD (0x1002)

  Device ID: 0x6920

  Revision ID: 0x0001

  ROM Revision: 113-C905AA-799

  VBIOS Version: 113-C905A1-007

  EFI Driver Version: 01.00.799

  Metal: Supported, feature set macOS GPUFamily2 v1

  Displays:

iMac:

  Display Type: Built-In Retina LCD

  Resolution: 5120x2880 Retina

  Framebuffer Depth: 30-Bit Colour (ARGB2101010)

  Main Display: Yes

  Mirror: Off

  Online: Yes

  Rotation: Supported

  Automatically Adjust Brightness: No

I changed the resolution of my built-in retina screen from Default to "More Space" (you can't actually choose a resolution as far as I can tell) and it seems to be stable now.

System Preferences - Displays - Display tab and Scaled radio button

I have been running in this configuration for a couple of hours now with Outlook open and it's been fine so far.

I'm going to run this way as my default.  I'll report back if the problem comes back or in a few days with a status update.

This worked for me.  And yes you can’t choose a specific  resolution just more space.  On other Macs you can be specific. 

I came across this article.   Another workaround might be to set up Outlook to run in Low Res mode.  See article below:  I will try this and see how this goes.. 

https://support.apple.com/en-gb/HT202471

Using apps with a Retina display

If an app looks different than you expect on your Retina display or high-resolution external display, try opening the app in low-resolution mode:

1. Quit the app.
2. Open the Applications folder.
3. Click the app once to select it, then choose Get Info from the File menu.
4. From the Get Info window that opens, select the checkbox labeled ”Open in Low Resolution.”
5. Close the Get Info window and open the app again.

Some apps that work best in low-resolution mode or that work only in low-resolution mode will have this mode already turned on, and in that case you might not be able to turn it off. The app developer might offer an update that includes support for the Retina display.