Different Types of Oak
To make life more complicated, there are plenty of differences in types and quality of Oak. A huge amount depends where it is grown, not just for the climate, but also the actual specie of oak.
In general terms, the finest oaks are Northern Appalachian White Oak from the USA, and French Oak. The American White Oak is prettier, harder, and has a more delicate grain structure than the French Oak, but French oak has a slightly more consistent, although duller colour, but of course some of these attributes just come down to personal preference.
The next grade down is either southern / eastern European oak or southern USA oak. In both cases the grain is bigger and much less attractive, with more colour variation. It is also softer, often significantly, and more prone to rot, being a much faster growing tree. In 40mm oak worktops, these are the predominant materials as they combine cheaper raw material costs with a OK, if not special finish. Certainly it is better to chose northern Appalachian oak over unspecified oak if the prices are similar and the suppliers are good.
At the bottom of the pile is Chinese or Russian oak. This tends to be very dark timber, often streaky looking because of minerals, with coarse grain and many more defects as the trunks are so much smaller. Not surprisingly, these form the bulk of the cheap end of the market, particularly 20mm stave worktops. Typically the oak for these trees has come from Siberia where the oak trunks measure only a couple of feet in diameter. The problem lies not just with the quality of this oak, but with the way it is dried. LINK
In recent years, 'African Oak' has started making an appearance, which has been interesting to watch as there is no such thing. It's one of many misleading names to timbers from Africa which are generally illegally logged. Anyone who sells 'African Oak' is misleading their customers at best.