A little history of Kush marijuana needs to be understood before an answer to your question can be answered. Kush strains of marijuana originated in the Hindu Kush region of Afghanistan and Pakistan. These were traditionally hash making strains, often cultivated by families for generations. The Kush strains from that region made it back to America and Europe in the 60's and 70's as people traveled across the "Hippie Trail". So trualy anything that does not decend from seed stock coming out of the Hindu Kush region is not Kush; this is where things get a bit confussing.
People have named and renamed strains with the the intention of selling the product quicker or for a bit more money. Strains like Hindu Kush and Master Kush definitely decend from Hindu Kush region seed stock. Other strains like OG Kush may not have any Kush genetics in them. This does not mean they do not have Afghanica lineage. There is currently a Kush craze that was created and is centered in CA and has spread out from there. So many people have put the name 'Kush' on many Indica and high potency plants so they may sell it quicker or for some extra money. So that 'Kush' you may be smoking just might not be; this doesn't mean it is inferior marijuana though.
As for OG Kush specifically, the lineage of this plant is rather unknown, and is surrounded by myth and lore. Many stories exist that all have different explanations as to where OG Kush Came from. The only thing that one can be certain of is that OG Kush is a decendant of the Chem/Dawg line of strains. But there is little known about the lineage of Chem/Dawg as well, making tracing any lineage close to impossible. So there may actually be Kush genetics in OG Kush, but there may not be.
When OG Kush finally made it out to the general masses it was hailed as a Holy Grail of herb. So many people wanted to figure out ways to have their very own version of OG Kush. So people started to rename cuts and cross out cuts of OG Kush to other plants, and thusly there are many versions of OG Kush. Each OG actually seems to be different from each other, meaning finding the true OG Kush is that much more difficult. So although they are all OG Kush and share many of the same general characteristics, each version has its own subtle differences.
I hope this was of some help.
