• obol (Greek obol) = 11.2 grain (mass)
• octant = 0.25 pi*rad (plane angle)
• octennial = 8 yr (time)
• octillion = 1E+27 (prefix)
• Oe = oersted (magnetic field strength)
• oersted = 79.5774715459477 A/m (magnetic field strength)
• ohm = V/A (electric resistance; derived unit)
• oil arroba (Spanish oil arroba) = 3.32 gallon (volume)
• oil barrel = petroleum barrel (volume)
• olympiad = 4 yr*approx (time)
• omega = ohm (electric resistance)
• omer (Israeli omer) = 0.45 peck (volume)
• one = 1 (prefix)
• ounce = "oz"
• oz (avoirdupois ounce) = 0.0625 pound (mass)
• oz = replacement for "ounce"
• oz ap (apothecary ounce) = 8 dram ap (mass)
• oz fl = oz fluid (volume)
• oz fluid (fluid ounce) = 0.25 gill (volume)
• oz t = oz troy (mass)
• oz troy (troy ounce) = 480 grain (mass)

A B C D E F G H I J K L M N O P Q R S T U V W X Y Z