You are probably not going to have much success trying to get a second stick of RAM to work in your system. Pairing off odd sticks of RAM just doesn't seem to work often, especially on faster boards, and particularly when you're trying to overclock them. What is the fastest, non-overclock speed your board will support? In your situation, I would buy 2x8GB RAM sticks that run at the fastest speed your motherboard supports non-overclocked (sell the old RAM on eBay--even used RAM is bringing close to new prices these days). Buy a top-end, name brand set: Corsair, G.Skill, Kingston.
One last thing. The general consensus is that overclocking RAM doesn't do much to improve the performance of your kit, and RAM is much touchier when it comes to overclocking than a CPU or GPU. It's pretty easy to damage it. Actually, overclocking in general is more of a game of chicken than a real-world way to improve the performance of your computer. I've done it, and it's fun. But I won't run anything overclocked more than it takes to see if it's stable, then it's back to stock settings. Save the wear and tear, and shortened life, of overclocked components. If you want a faster system, buy faster parts.
That's my opinion. Others will disagree. Your mileage may vary.