These are my general recommendations for getting to sub 20 OH.
First, you will likely see the best results from CFOP from the information you've given. And you will want to aim to have your 2H times around sub 15 to sub 11 in order to get sub 20 OH. This is because OH times are generally 1.5x-2x longer than your 2H times, depending on how much you dedicate to the event.
You want to make sure your fundamentals of turning are extremely well-practised. The videos linked by
@Fred are very good, as is
this video by OH Cuber, for another perspective on fingertricks.
I would also recommend at least learning full OH PLL, as you will be able to save a lot of time by not having to do two long algorithms. I would use
this sheet, as it has all of the best OH algs, as well as multiple alternatives in case you don't like the main alg.
You can also learn full OLL using
this sheet, which also has the best algs as well as alternatives, but at least two-look OLL will suffice. And for two-look OLL, you just want to be able to reduce any case into one of seven OCLLs, the cases with a cross already made. For any of the line cases, just hold it horizontally and do F R U R' U' F'. For the L cases, hold it in the top left and do F U R U' R' F', or alternatively, hold the L in the bottom right and do f R U R' U' f'. And finally for Dot cases, do f U R U' R' S' U R U' R' F'. The S move fingertrick can be learnt from any of the above fingertrick guides, but S' is much easier for beginners than S, so there shouldn't be much trouble learning it, but if it still presents a problem, then you can do f' F, which is slower but makes fingertricking it easier.
You also won't need to have very high TPS to reach sub-20, only around 3-4 TPS depending on how efficient you are. So in order to get your solutions more efficient, I would recommend watching a lot of example solves from high-level cubers to see how they handle certain cases. You can use OH Cuber from before to find some of those example solves, as he has a few videos ranging in different speeds.
You should also take advantage of your slower OH turning to have much better lookahead. Since you will be turning slower than your 2H speed, you will be able to see how the pieces are being affected much more easily and thus can capitalise on it better. This will just need to be something you pick up as you start doing more solves and get more accustomed to OH turning, as it can be rough to lookahead to start.
I would also recommend getting a good cube for OH in case you don't have one already. There are a few options to choose from, but the main two cubes I would recommend are the RS3M V5 Ballcore UV. This is a cheap cube while also providing excellent performance and is used by a majority of the top OH solvers. I would recommend not dropping it, however, as the plastic can be brittle and chip at the corners if you happen to drop it. The second cube I would recommend is the WR M V10 Ballcore UV. This is likely one of the more used cubes and my personal favourite, as it has similar performance to the V5, but it feels more stable and blocky with slightly stronger magnets. There are also many versions you can get, ranging from the original to the Ferrocore to the Picube versions. Each one has a unique feel, and there isn't really a definitive way to find out which one would be best for you, but if you are worried about the core breaking, then the Ferrocore and its metal core will likely be the best.
There are also a few channels I would recommend you check out, as they have a lot of good OH content you can watch to get a better feel for the event.
The first being
OH Cuber, like I've mentioned above, as he is one of the best OH solvers, and he has made quite a few videos sharing his insights into the event, with example solves, tutorials, guides, and more that you may find helpful.
I would also recommend
BrodytheCuber, since he has a ton of videos spanning all the different aspects of OH. They may be a bit older, but I still find them to be extremely helpful. He also has a few breakdowns from some fast solvers, going in depth about their unique solving style, and it may give you some extra insight into different ways you could be solving. You may also be interested in t
his video where he breaks down Zaiyang Zhang's solving, which uses almost no table abuse, similar to what you want. Though as you will see, he uses some table abuse, which is something you should consider as well. The table isn't your enemy, and it can be helpful in getting the cube to quickly realign itself if it ever gets misaligned. I would also suggest you watch as many of Brody's OH videos as you can, since they are some of the best-quality videos on OH improvement I have personally come across.
I can also offer up some more personal help that none of the above examples can. If you need any help with specific cases or struggles, then I am more than willing to provide it; just send me a DM here and we can work something out. I may not be as quick as some of the resources above, but I'm nearly sub-11 OH, and I have a considerable amount of knowledge from having OH as my main event for three years now. Hopefully all of this information helps you on your journey to sub-20 OH.
