Upgrading RAM on an old Optiplex is usually safe — if you check the specs first.

Yes, but you have to be careful about what you buy.

Proprietary motherboards in pre-builts like Dell Optiplex mostly use standard DIMM slots physically, but the BIOS can be picky about RAM. The main things to watch: max capacity (many older models top out at 16GB or 32GB), voltage (some need 1.35V low-voltage DDR3L, not standard 1.5V), and whether they require non-ECC unbuffered memory (most do, server RAM will not work).

Physically installing the wrong RAM won’t fry the board — it just won’t boot, or it’ll run at reduced speeds. So “safe” as in no explosion, yes. But you might waste money if you buy the wrong sticks. Check the service manual for your exact model (Dell’s site is fine) and stick to Crucial or Kingston’s compatibility tools to be sure.

Still the cheapest speed boost you’ll get for that old box.