os

oscillate (v.) to sway from one side to the other (My uncle oscillated between buying a station wagon to transport his family and buying a sports car to satisfy his boyhood fantasies.)

ostensible (adj.) appearing as such, seemingly (Jack’s ostensible reason for driving was that airfare was too expensive, but in reality, he was afraid of flying.)

ostentatious (adj.) excessively showy, glitzy (On the palace tour, the guide focused on the ostentatious decorations and spoke little of the royal family’s history.)

ostracism (n.) exclusion from a group (Beth risked ostracism if her roommates discovered her flatulence.)