OnS isn't recommended for its patterns, rather, it's recommended because it's basic and it's simple.
When you see newbies go it on their own without patterns, they generally go either one of two routes, either they end up with some regretsy-tier Yumi King disaster, or else they're excellent seamstresses but really new and end up with an over-designed stiff rufflemonster costume that probably took hours to make but nobody wants to look at it because it's so ugly. Or they end up with the Simplicity pattern and the results always seem to end up in the ita thread.
OnS just solves a lot of problems. The designs are basic but not ugly messes. The construction is easy, if you can read Japanese you can start with it, it's only because most people here can't that it's recommended you put together some other clothes before using OnS. You can sew entire coords, even mini wardrobes in some of the volumes. If you have a Japanese shopping service, it even tells you what fabric they used, so you can copy the outfit exactly.
Slopers are good, only thing to watch out for is keeping an eye on proportions, because lolita has a lot of extra fabric, eg- poofy skirt, puffy sleeves, puffy shorts, trapeze dresses, etc.