There is only one file type that uses the .OTB extension:
Nokia Over The Air Bitmap Image
Over The Air (OTA) image used for transmitting pictures between Nokia and Siemens mobile phones; stores a black and white bitmap image containing a maximum of 255x255 pixels; may be saved in a hexadecimal text format or a binary format.
OTB files are transmitted as a series of one or more SMS text messages.
NOTE: Nokia Multimedia Player has been discontinued and is no longer available for download.
Updated: April 23, 2016