Like all domestic species, solid Oak flooring is given one of several grades: Select & Better, #1 Common, #2 Common, #3 Common (also known as Utility or Cabin Grade), and Character. While all Oak flooring has the same qualities, the grade it is assigned per standards governed by the National Wood Flooring Association (NWFA) indicates its appearance. As such, an even appearance receives a higher grade, while hardwood with character marks or color variation is marked lower.
Select & Better Oak flooring is primarily heartwood and has a uniform color. Select Oak flooring also contains more sapwood than lower grades.
Moving down the grading scale, #1 Common flooring, on the other hand, retains some of these qualities but may start to display a small amount of character marks.
#1 Common, #2 Common, and Character floors also have more color variation, including streaking and knots.
If a floor that displays an overall even appearance is your preference, consider going with a White Oak or Red Oak Select & Better floor.
Keep in mind that, regardless of the amount of character marks Oak hardwood has, each species has a general set of qualities.