Range of OHI is:
The Oral Hygiene Index (OHI) was developed in 1960 by John C. Greene and Jack R. Vermillion to classify and assess oral hygiene status.
To calculate the OHI, the DI and CI are summed: OHI = DI + CI 
The DI and CI values range from 0 to 6.
OHI value ranges from 0 to 12.

Essentials of preventive and community dentistry
Soben Peter
5th edition
page no. 318
0-4
3-Jan
0-6
0-12