I went to Siam Oishi a few times when my favorite local Thai place was busy (weekends, long wait).
Siam Oishi Thai food looks and tastes like the "tourist" version of Thai food they serve in Thailand to "farangs" (that's "foreigners"). It's neither particularly good nor terribly bad. It provides a short term "fix" when other options are unavailable.
Service is fine, probably a bit better than in the "local" Thai place, as is decor. Prices are slightly higher than basic Thai places, but would be just fine if food was better.
However, on a Friday night, this restaurant is half-empty. Make your own conclusions. Personally, I might go back if the "local" place is too busy again, but will not make it destination by itself.
This review is the subjective opinion of a TripAdvisor member and not of TripAdvisor LLC.