I never used cilantro oil. I tried cilantro tablets by noticed no improvement, but I only did that a few weeks which may not have been enough time. I switched to fresh daily for about 4 months, then learned about how frozen was also helping and then used both fresh and frozen. Using frozen made the smoothie so much more convenient. I started noticing definite results with fresh cilantro in less than a month.

Its purchased on Amazon, called Herb Pharm, there is no strength, one full squeeze of the dropper 2-3 times a day with a little bit of water. It seems to work pretty well for me.

I use the same Cilantro drops as @nodgabnoj , but not nearly that amount. Here is a link to this product on Amazon, but you can buy it directly from Herb Pharm too.

I started out very slowly concerning the number of drops I used after reading the reviews of the product further down the page on Amazon. Using the amount recommended on the product label resulted in some people detoxing too fast and they got very sick. I started out with only one drop twice/day and gradually worked my way up to larger amounts. I'm sure it will vary from individual to individual how much can be tolerated. Also, don't forget to shake the bottle well before each use.

Finally, @gardeningjunkie and others on this website, and the other website she references, got their results from using regular cilantro you would purchase in a grocery store and then processing it for using in a smoothie. Although it apparently works for only about half the people who try it, there is far more anecdotal evidence for this method of using cilantro for GD, so if you don't mind going to a bit more bother, this might be the way to go for you.
Good luck and keep us posted on what happens.