Stanford OHS has some of the highest scoring kids in the country.
I would think that most kids who submit ACT scores would only submit 27+. Anecdotally I know a number of kids who submitted 1500+ SAT scores when applying for 9th grade (including my own, who takes a single course at OHS now).
If you do not think you can score 27+ on the ACT, I think I would lean towards the SSAT or ISEE and target 90%+ overall percentile scores. Many kids will get in with lower scores, of course, but I think above 90% the scores tend to have a very positive impact on your admissions chances.
OHS is not super selective by admissions percentages (roughly 30-40% are admitted, although some are conditional or limited admissions), but the school does tend to attract an applicant group that is very strong academically.
Best of luck!