Alright, I just read this whole thread and I think I am having the same problem as you are Kratose. I currently have an OG Kush mom that is having the same symptoms, grayish spots on yellowing leaves. After searching all over for info on OG Kush grows, I came across a video on youtube from attitude seedbank. They had a behind-the-scenes with Emerald Triangle video where one of the growers was discussing this issue.
http://www.youtube.com/watch?v=aqiPgUM4VZo is the link to the video. He is talking about OG Kush strains or strains that contain OG Kush and he mentions that strains with OG Kush genetics are typically VERY AGGRESSIVE FEEDERS. He mentions that OG Kush always requires a lot of N-P-K compared to other strains. If looking at your pictures, and assuming this is true, I believe you have multiple symptoms. It looks like your plant wants more of all three, slight yellowing could be related to some nitrogen and/or potassium shortage, and grayish 'dry-oil' spots on leaves could be related to phosphorus deficiencies. Seeing that the plant eats a ton, I suspect you may want to use a very complete NPK fertilizer, and use it at moderate strength. If yellowing goes away and spots stay, try buying some chelated phosphorus or some other form of phosphorus by itself so as not to create nute lockouts by over feeding with the Growbig. As you feed with the complete fertilizer, you may correct one problem and lock out nutes creating another, often when troubleshooting, using the individual ingredient needed to correct the problem is the safest way to ensure fixing one problem without creating a handful more. I'm currently using Earth Juice Grow at a somewhat weak concentration on my OG Kush mom and her 'oil spots' and yellowing are fading away...earth juice grow is surprising a 2-1-1 fertilizer, and is surprisingly strong and fast acting. The mom isn't completely healthy yet, but she's on her way back. Be sure to lay off the micro-nutes until you see an issue with deficiencies as OG Kush appears to have a reputation of eating a lot of NPK but not necessarily eating a lot of the micros. Hope this helps...as my problem isn't completely gone, I can't guarantee I'm correct, but I don't think anyone has given this opinion yet so I figured I would
Best of luck with the OG Kush, if you can save her, she's amazing, huge yields, crazy potent and pungent, with sight smell and taste appeal...however, if you flower or clone while she's pissed about some nutrient problem, prepare to struggle, best to operate with this girl when she is green and growing. I did notice putting her clones into flower seems to create an even larger P-K shortage, but with Earth Juice Bloom and other additives I like, it seems to have enough to cover it. Even Fox Farm Tiger Bloom seems to have enough P-K to prevent the deficiencies...but I prefer organics
