I would not read too much into results from a group screener. From what I understand, individual IQ tests are more accurate. The unfortunate thing is that, despite the fact that group tests like the CogAT and OLSAT are not supposed to be IQ tests, some schools use them that way (and some teachers and administrators don't seem to know the difference).
I know less about the OLSAT, as my kiddos have not taken it. I seem to have one of the rare DC whose CogATs and IQ results were not in stark contrast (both were 99 or 99+ percentile composites; DD has taken CogAT twice). Still, I get the sense that group tests often fall LOWER than individual IQ results. To be fair, the CogAT version DD took had a lower ceiling than did the IQ test (which had extended norms). DD's FSIQ (on the WISC) was 8-11 points higher than her CogAT composite, and her GAI (WISC) was 21-24 points higher.
Also adding this info from Hoagie's:
Group intelligence tests are commonly used as screening measures, to see if the child should move to a full gifted assessment. They are commonly administered by teachers. Group tests are generally normed on populations of all children, with relatively few gifted children among the mix. When taking group intelligence tests, gifted kids often "over-think" the questions, and perhaps make wrong selections. And since there's no individual tester to clarify unusual answers, the gifted kids often score lower on group intelligence tests.
The most common group intelligence tests, OLSAT and CogAT, are used in districts and programs across the country. Notable gifted professionals recommend them for screening potentially gifted children. However, a small study noted a potential problem with the OLSAT and very gifted children. While the correlation between group and individual intelligence tests is quite high for average scores, in this study that correlation almost disappeared for gifted scores. This means that while an average child will score very similarly on a group IQ test and an individual IQ test, a gifted child may not score similarly at all. And the study suggests that this group test may even result in a negative correlation for some gifted children: the more gifted the child, the lower the group ability test score! ["Investigations of the Otis-Lennon School Ability Test to Predict WISC-R Full Scale IQ for Referred Children" by Anna H. Avant and Marcia R. O'Neal, University of Alabama, Nov. 1986, ED286883] Though this study is no longer available from AskERIC, it can be obtained on microfiche from most education university libraries.
A 2001 study using the OLSAT noted a problem with the OLSAT and twice exceptional (gifted and learning disabled) students. A Comparison of the WISC-III and the Otis-Lennon School Ability Test with Students Referred for Learning Disabilities, by Thomas Guilmette et. al., Providence College and Brown University School of Medicine, showed that LD kids tended to score an average of 7.5 points lower on the OLSAT than their WISC-III full scale IQ scores. This study is available in the Journal of Psychoeducational Assessment, or for a few dollars from SAGE Publications on the 'net.
"The use of the OLSAT-6 in estimating overall intellectual abilities in children with suspected learning disabilities is not encouraged because it may frequently underestimate students' actual abilities, which may result in fewer appropriate referrals for further educational and intellectual abilities." "As with previous studies with gifted students, our research indicates that the OLSAT-6 appears to underestimate WISC-III FSIQ." -- Guilmette et. al., A Comparison of the WISC-III and the Otis-Lennon School Ability Test with Students Referred for Learning Disabilities
Individual intelligence tests are considered the most accurate measure of intelligence, but even they are not perfect. The Wechsler tests, Wechsler Intelligence Scale for Children (WISC-IV) and Wechsler Preschool and Primary Scale of Intelligence (WPPSI-III), and the Stanford Binet (SB-5) are the most common individual IQ tests. The WISC and WPPSI are most commonly used by schools. The Woodcock-Johnson III (WJ-III) cognitive, a relative to the WJ-III individual achievement test, may also be used. Individual IQ tests must be given by a school or counseling psychologist.
