I complied iso2opl on my Mac with no errors (thanks to jimmi's suggestion). It was actually simpler than I thought.
Works great! I tested it with Socom 3, which is over 4GB, with no problems.
I can post mine so you can compare to yours to see if it was your iso or not.
PS2 V7 NTSC w/ FMCB 1.8b
Maxtor 200GB HDD
Official Sony 40GB HDD
uLE 4.42/HDL 0.8c/OPL (current build)/GSM 0.23x
1 Sony PS2 8MB Memory Card
1 Katana 4x8MB Memory Card
Custom PS2 Case with matching keyboard, remote, and controllers