Organizational Behavior Management could be concieved as a subfield within I/O Psychology but I think of it as more of a parallel approach. OBM focuses on solving organizational problems primarily through the use of contingency management, although there are other techniques employed.
For an excellent article of comparing OBM with traditional I/O psychology, check out this reference:
Bucklin, B.R., Alvero, A. M., Dickinson, A. M., Austin, J., Jackson, A. K. (2000). Industrial-organizational Psychology and Organizational Behavior Management: An Objective Comparison. Journal of Organizational Behavior Management, 20 (2), 27-75.
For a briefer answer check this link:
What is OBM?