Focusing on strangeness:
* As odd as a left sock in a dryer full of right socks.
* As odd as a square wheel on a bicycle.
* As odd as a cat wearing a monocle.
Focusing on uniqueness:
* As odd as a snowflake in a blizzard.
* As odd as a single star in a vast, dark sky.
* As odd as a lone pebble on a beach.
Focusing on being out of place:
* As odd as a penguin in a desert.
* As odd as a fish out of water.
* As odd as a banjo in a symphony orchestra.
The best simile will depend on the specific context and the desired effect.