1. Vigilant
   - on guard, upon one's guard, on your guard

See also: wary

• Oligoporus leucospongia
• oligopsony
• oligosaccharide
• oligospermia
• oligotrophic
• oliguria
• Olimbos
• olio
• olivaceous
• olive
• olive branch
• olive drab
• olive family
• olive oil
• olive ridley
• olive tree
• olive-brown
• olive-colored
• olive-coloured
• olive-drab uniform
• olive-gray
• olive-green
• olive-grey
• olive-sized
• olive-tree agaric