Yes, the RX 550 should work fine in your OptiPlex 980 MT.
Hello, I hope you all are doing well! My loyal Optiplex 980 MT of over 10 years is still working with a Radeon HD 6570 1 GB graphic card. I want to replace it with a 4 GB card. I found the ASUS Phoenix AMD Radeon RX 550 Graphics Card PCIe 3.0, 4GB GDDR5 and was wondering if it would run on my Optiplex 980 MT?  Sincerely and regards,
Optiplex 980 MT and AMD Radeon RX 550 4GB?