The 1907-O Quarter Dollar is a common coin in all grades. Mint State examples are plentiful, but not nearly as plentiful as some of the dates with thge largest mintages. Among Mint State grades, MS64 appears most often. In Gem condition, the 1907-O Quarter becomes very scarce, though more than two dozen examples are known in MS65 and MS66 combined. A small handful of MS67 examples exist, most of which are colorfully toned (see above). The best example is a single NGC MS68, but the price record belongs to a PCGS MS67 from the Eugene Gardner Collection sold in 2015.
The mintmark is never directly under the center of the eagle's tail. Instead, it is below the left side (viewer's right) of the tail, sometimes closer to the R and sometimes closer to the D.
David Lawrence Rare Coins, sold privately in 10/1999 - Eugene H. Gardner Collection, Part IV - Heritage 10/2015:98587, $10,575