Here's a breakdown:
* OH: Like the "o" in "go"
* stahr: Like the "star" in "starlight"
* uh: Like the "u" in "cup"
However, there are some variations depending on the region and the speaker's preference. You might also hear it pronounced as OSS-tahr-uh or OSS-trah.
Ultimately, it's up to the individual to decide how they pronounce it, but the most common pronunciation is OH-stahr-uh.