OTR is the short name for Off-the-Road.

OTR tire range usually refers to tires used on rough terrain applications.Applications:
Crane service
Port service
Construction service
Mining
Specialty service

Tire sizes from 24inch to 35inch are usually considered as OTR tires.
Here is a list of the sizes available with us.
12.00R24, 13.00R25, 14.00R24, 14.00R25, 16.00R25,18.00R25, 18.00R33, 21.00R33,  21.00R35,  24.00R35
15.5R25，17.5R25，20.5R25，23.5R25，26.5R25，29.5R25，29.5R29
600/65R25，650/65R25，750/65R25， 875/65R29, 35/65R33

Tire sizes of 49inch or above, are usually considered as Giant OTR tires.
45/65R45, 27.00R49, 30.00R51, 33.00R51,  36.00R51, 40.00R57