The 1857-O Half Dollar has a relatively hefty mintage, yet it remains very scarce to rare in any Mint State grade. Collectos simply did not pay attention to this date "back in the day." PCGS has certified several MS64 example, but none finer. NGC reports a single MS66 example (from the Richmond Collection), but that one last appeared on the market in 2006 and has not reappeared since.
The strike on 1857-O Half Dollars ranges from very flat to reasonably strong. When possible, collectors should seek out examples with sharp details and clean surfaces.