You can use FMCB_CFG.ELF (should be installed into mc?:/SYS-CONF/ where ? is 0 for slot 1 or 1 for slot 2) to assign ELFs to a pad button, which you can hold down during FMCB boot logo until screen goes black, and it will launch the assigned program.
Inside FMCB_CFG.ELF, go to "Configure E1 launch keys...", go down to the desired button, hit square (for "map to any MC" which means it will work whether you have your MC in slot 1 or 2), navigate to where your OPL ELF is stored (mine is mc1:/BOOT/OPL.ELF as I keep my FMCB card in slot 2), go down to "Return" at the bottom of the list of buttons, then choose either "Save CNF to MC0" or "Save CNF to MC1" depending on which slow your FMCB card is stored. Then, after it saves, select "PS2 Browser (FMCB Restart)". When you see the FMCB logo, hold down the button you assigned to OPL and when the screen goes black, OPL will load. I also do this with uLE so that either is accessible as a sort of "manual auto boot" type deal.
Another option is to use FMCB_CFG.ELF to add an entry for OPL to the FMCB hacked OSDSYS menu. It works similarly to assigning pad buttons. You go to "Configure OSDSYS options...", scroll down to "Configure Item 1 :" (which is likely listed as uLaunchELF if you have the default FREEMCB.CNF), press right on the d-pad until you find a blank slot or a slot listed as a program you don't use and then select it, select "Name:" and use the onscreen keyboard to enter the title you wish to use (I use the full title "Open PS2 Loader" here but you can put OPL if you don't feel like typing it all out with a PS2 pad), go down to "Path1:" and hit square, then navigate to your OPL ELF. You can also use Path2 to map it to an external copy stored on a USB flash drive, and if something causes the ELF stored on the MC to be deleted, then FMCB will search for the ELF on the flash drive and if it is found, it will still display it in the menu. If something happens to the MC copy of OPL and the flash drive is not inserted, then OPL will not show up on the OSDSYS menu. Return to the main FMCB_CFG menu and save it in the same way as before, and now OPL will show up on the OSDSYS menu when you boot from your FMCB card.
Hope this helps
Chip-free homebrew <3
SCPH-39001, FMCB 1.8c, NA+250GB HDD
SCPH-75001, FMCB 1.8c
GSM 0.36 + OPL r639 = Gaming bliss ^_^