"I'm thinking of things that begin with the letter . ." O

          Octopus
        Orlando
         Orangutan
          Oneida
            Orange
          Obstacle
        Olympics
     Oracle
   Onto
  Ollie
   Olivier
    Omnipotent
      Oscillating
        Oscar
          Ornate
            Open
              Occipital
                Obsessive
                  Obtuse
                 Occupation
               Occult
             Outward
           Odd
         Ocean
       Offended
        Ogden
          Ohio
             Oily
               Okay
                  Olay 
                    Omitted
                       Oneness
                           Oops!
                             Opportunity
                                O'Quinn
                                   Oregon
                                   Oregano
                                 Ostracized
                               Organization
                           Otter
                        Out
                     Oven
                   Own
               Owls
             Oxen
          Oysters
             Oz