Opera is a fork of 4DO, originally a port of 4DO, itself a fork of FreeDO, to libretro. The fork/rename occurred due to the original 4DO project being dormant and to differenciate the project due to new development and focus.

One of several BIOS ROMs must be placed into your RetroArch / libretro "System Directory" folder. "norsa" versions have the RSA encryption check disabled which should allow unsigned software to run.

For certain Japanese games a second "Kanji" ROM is necessary.

Compatibility lists:

• fourdo.com
• 3dodev.com