There is a body of research that has found that the OLSAT is only moderately correlated to the WISC.

About half-way down this page is a small pilot study that suggests that this finding continues to be the case for the OLSAT-8 and WISC-IV:

https://www.ucmo.edu/osp/reports.cfm

Previous research on the OLSAT-6 and WISC-III:

http://jpa.sagepub.com/content/19/3/239.short
http://cjs.sagepub.com/content/11/2/120.short (this one specifically about GT identification)

The same phenomenon is common enough with other group aptitude tests that the publisher of the CogAT actually released a consumer info sheet on it for the CogAT-6:

http://www.riverpub.com/products/group/cogat6/pdfs/newsletters/CS_vol1_summer04.pdf

...pronounced like the long vowel and first letter of the alphabet...