RB-551 was an OOM-Battle Droid used by the CIS that was a part of a battle droid squad during the Battle of Ryloth. He was scouting a small area that was scrapyard of damaged Republic and Separatist tanks when his squad was attacked by a Twi'lek rebel group led by Charm Syndulla and was destroyed. his head was found by another squad of battle droids, who made fun of the droid for being an old OOM battle droid and not a newer B1-Battle Droid unit like they were, but these droids shared RB-551's fate when they were also destroyed by Charm's rebel group as well, with their remains were found by Jedi master Mace Windu and his clone ARF troopers.