Oumbärlig, indispensable.
Oundviklig, inevitable.
Oupphörlig, continual.
Ouppmärksam, inattentive.
Ouppriktig, insincere, false.
Ouppsåtlig, unintentional.
Oursäktlig, inexcusable.
Outforskad, unexplored.
Outförbar, impracticable.
Outslitlig, not to be worn out.