The 1857-O Half Dime is a relatively common coin that seems to have survived better than some of the earlier O-Mint dates from the 1850s. It is not nearly as common as the 1858-O, but it is valued higher than the 1859-O (which has a smaller mintage).
A small number of Gem examples exist which skew the Condition Census up into the MS66 range. The finest example is a single PCGS MS68 from the Simpson collection. Strike quality remains an issue on this date but there are some well-struck (not full struck) examples that show stronger denticles than usual.