To use localization in theme mode you have to create a folder:
and put your *.mo files there.
Secondly, you have to fix a small bug in OT itself:
open the file ot-loader.php, and find the line that reads:
define( 'OT_LANG_DIR', dirname( plugin_basename( __FILE__ ) ) . '/languages/' );
and replace it with:
define( 'OT_LANG_DIR', '/' . dirname( plugin_basename( __FILE__ ) ) . '/languages/' );
The reason for this is, the function "plugin_basename" trims any and all slashes (both leading and trailing) from the path and without the added slash OT_LANG_DIR can never be found.
I've reported the issue on github: https://github.com/valendesigns/option-tree/issues/200
EDIT: typo, had 1 closing parentheses too many