1. Not observed
   - unseen 
2. Not observed
   "managed to slip away unobserved"

See also: undetected, unnoticed

• Olfersia
• Olfersia cervina
• Olga Korbut
• olibanum
• oligarch
• oligarchic
• oligarchical
• oligarchy
• oligo-
• Oligocene
• Oligocene epoch
• Oligochaeta
• oligochaete
• oligochaete worm
• oligoclase
• oligodactyly
• oligodendria
• oligodendrocyte
• oligodendroglia
• oligodontia
• oligomenorrhea
• oligonucleotide
• Oligoplites
• oligopoly
• Oligoporus