The documentation for Strata is here but it doesn't include a comparison with OG-Platform. The key difference is that Strata is a Java library, available in Maven Central, whereas OG-Platform was a large scale server that required lots of setup, such as databases. Where OG-Platform defines the databases and external code to call Bloomberg et al, Strata simply expects callers to pass that data in. If you are only using the analytics, Strata is much simpler and cleaner to work with - fewer dependencies and more thoughtful design.
Hope that helps