You can install a modchip or use a softmod (FMCB, works on most models).
If you're looking for a modchip I think the Modbo chips are the only ones still easily available nowadays. You'll of course need a soldering station to install it:
https://quade.co/ps2-modchip-guide/modbo/
With FMCB you can't play PS1 games using the built-in BC and only PS2 burned discs after patching them. But if you only want to use OPL to load from USB/LAN/HDD then FMCB is enough.
For some models you can use a DVD to install FMCB:
https://www.psx-place.com/threads/tutorial-fmcb-installation-with-free-dvd-boot.30211/
Otherwise you'll need a pre-modded unit to install it to your memory card or buy a card with it pre-installed.