Um so here's the deal um someone came over to my house and decided they'd upgrade my PS3 fat system firmware without permission so they could play online. Only problem is I had a pre-firmware version 3.21 firmware installed and now that he's installed firmware version 3.55 my "Other OS" option is effed up. Does anyone have any ideas? Any Custom Firmware that allows other OS (it'd also be cool to add other OS to my firmware 3.55 Slim PS3 )? Any way to re-flash it to factory? Anything I'm desperate. I don't care which PS3 has the Other-OS feature on it I just need one to have it the slim or the fat.

Boned!!!!!
Other OS is not fucked up, Sony decided users do not need it (a possible way to play boot legs or install the games on the hard drive based fear from Sony).
PS3 Slim with "Other OS": At this current moment in time non-existent.
How do I downgrade the firmware: Have you tried Googling it? Find the sties that mention Jailbreak and get one of the devices and programs. Then you can downgrade your Fat Ps3.

Sony is doing the same thing that they have been doing for the PSP- release a new patch that prevents the use of a Jailbreaking device and, hackers compromise it shortly thereafter. (repeat till Sony stops making new patches)

Ps- this does answer the question you asked. Its not my fault you do not like/can't get the anwser at this time. Check yourself!

This however does not help me as I'm not able to get a jailbreak at this time Q.Q >v>