For what exact OptiPlex model ?

Normally, if there is a M.2-NVMe slot on the motherboard, Windows-bootable as C: is supported and preferred. Use UEFI-Mode and Windows Boot Manager.

https://www.dell.com/community/Alienware-Desktops/M-2-NVMe-bootable-options/m-p/6073081/highlight/true#M3401

9020

For the Dell OptiPlex 9020 SFF, I don't see a M.2 slot on the motherboard. Do you?

https://topics-cdn.dell.com/pdf/optiplex-9020-desktop_owners-manual_en-us.pdf

The PCIe x4 card-slot might support a bootable NVMe Add-In card since ... IIRC, this machine is UEFI.

Conventional 2.5inch SATA-3/600 is fast SSD too.

I was thinking of something like this? M.2 PCIe to PCIe 3.0 x4 Adapter

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@Kev7274 wrote:

I was thinking of something like this? M.2 PCIe to PCIe 3.0 x4 Adapter

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Right. That would be the only way.

After install ... if it appears in BIOS and is shown as an available Windows install location as C: , it should work for the rest.

For an old machine ... I took the easier/cheaper road ... $60 2.5inch Kingston SATA-3/600 SSD into a 9020 MT. Boots-up in under 15 seconds now and very "snappy" in general use.

Sounds like a plan. That should turn it into a useful usable machine again.

On a SATA-3/600 port, you should be getting reads around 500. IOPS off the charts better than a spinner.

I just got a standard ssd to use,seemed to be the easiest way to do it. 

My OptiPlex 9020 benchmarked an M.2 NVMe SSD at 1600 MB/sec. and an M.2 AHCI SSD at 1500 MB/sec. That's about three times faster than the fastest SATA SSD, and that's more than enough reason to fill up my otherwise empty PCIe slot, while keeping the OptiPlex's scarce SATA connectors for other mass storage devices.

Even with the latest available firmware installed (A24, Nov. 2, 2018). I cloned my boot drive to the AHCI SSD (which should still work as a legacy BIOS device), yet System Information shows the device only as "SLOT4= Mass Storage" and not as a bootable device -- even with the UEFI Boot Option enabled, and with or without Secure Boot enabled. I would like to see Dell update the next UEFI/BIOS to include support for bootable M.2 drives.

Just last month (Oct. 2018 as of this writing), even Apple updated its Boot ROM to support NVMe drives on my nine-year-old Mac Pro! Apple is notorious for ditching support of their older systems, yet they still provided this added feature to their users. Dell should follow Apple's lead in allowing its users to experience the blinding speed of an NVMe boot drive.

Booting in a UEFI environment requires the boot drive be partitioned GPT (MBR won't work).  You can check partition type with Windows Disk Management.  Right-clicking on gray Disk 1, Disk 2, etc. and selecting Properties, Volumes tab will show Partition style.  Anyway, worth double-checking...

Would using GPT on the NVME let it be used as a boot drive?

Generally GPT is required to boot from a NVMe SSD.  I haven't seen anyone boot a M.2 NVMe SSD on the OptiPlex 9020 SFF, but maybe everyone trying was using a MBR partition?

I may have to try and see what happens.

...I believe Dell shipped the 9020 with things set to Legacy BIOS and disk partitioned MBR.  If so, it would mean anyone cloning the original drive to a M.2 NVMe SSD would not be able to boot from it.