How to Transfer data from an dell sff oem ssd to a regular m2 nvme ssd?

clone the oem ssd to a portable hard drive then restore system on the new ssd.

https://www.dell.com/community/en/conversations/optiplex-desktops/how-to-clone-ssd-to-another-ssd-without-third-party-tool/652b338043dbba53b173f0b7?keyword=system%20image

Always include PC model and version of Windows in your posts.

What do you mean by "sff oem SSD"? Is it a 2.5" SATA SSD, an M.2 SATA SSD, or an M.2 NVME SSD?  Without the PC model, we don't know if the M.2 socket on the motherboard supports a SATA SSD or an NVME SSD or both...

You need imaging software to image the existing SSD and save the image on an external USB HDD, or on an internal HDD, if you have one and there's enough free space. The imaging software should also give you an option to create a bootable USB that you'll need to transfer the image onto the new SSD.

If the OEM SSD is in the M.2 slot, after it's imaged, you would swap in the new SSD and use the bootable USB to boot the PC and copy the image from wherever you stored it onto the new SSD. 

Keep in mind that if the new SSD has bigger capacity than the original, you will need to expand the size of the C: partition when you copy the image onto the new drive, so all the extra space can be used. So make sure you understand how the imaging software can expand the C: partition before you start...

Macrium Reflect is good imaging software and there's a free 30-day trial...

@RoHe​ NVME M.2 SSD is the one im talking about

@udayaSB​ its also a proprietary length, thats why its sff, and the im using inspiron 3891.

You still haven't told us what PC model this is.

If SATA Operation is set to RAID in BIOS setup now, you may want to reconfigure BIOS and Windows to use AHCI first because some NVME SSDs don't work well if BIOS is set to RAID. This has to be done the right way or you'll make the PC unbootable. So don't just change BIOS to AHCI now. 

Since you don't have 2 NVME slots in the PC, you can't clone the old NVME SSD directly onto the new one, unless you want to buy an NVME SSD enclosure for this process. 

So you'll have to create an image of the old SSD and save it on either an external USB HDD or internal HDD. Then swap in the new SSD and use the bootable USB stick you create with the imaging software to copy the saved image onto the new SSD, increasing the size of the C: partition, assuming the new SSD is larger than old one.

SSDs are all standard sizes. There's no Dell "proprietary length" SSD.  In general, SSDs with high(er) storage capacity are 80 mm long and ones with lower capacity are only 30 mm long.  

The Inspiron 3891 can use either an M.2 NVME 2230 or M.2 NVME 2280 SSD, where 22 is the width (mm) of the SSD and 30 or 80 is the length (mm).  (NOTE: There is a separate slot for only an M.2 2230 WiFi card in this PC model, but that slot only works with a WiFi card.)

How to replace the M.2 2230 solid-state drive on Inspiron 3891 (video)

And sff (small form factor) refers to the size of the PC case, and has nothing to do with the size of the SSD that's installed.