As a beginner, you are going to be prone to making a lot of mistakes. It is all part of the learning process though so you should never be ashamed of making a small mistake in the oil painting process. One way to avoid it would be to watch experienced oil painters do their thing. Aside from doing it on the Internet, it would be a lot better to do it in person, perhaps while the two of you are relaxing by the fire. In fact, you have all the freedom in the world to ask a ton of questions to the person standing right in front of you doing his thing. This person would be happy about asking him questions because it would contribute to the common goal of having more oil painters in this world.
It is possible you would have questions about the edges, subjects, or even shadows. Yes, including the shadow in the oil painting while you have something right in front of you would be a big mistake and while it may look nice in person, it won’t look good in a drawing. Thus, better set the subject in such a way that there are no shadows present for you to paint using oil. You must never bring too much coloring options with you whenever you start with the oil painting process. That is going to result in too many colors in your painting.
Unfortunately, that means it would look a bit too bright and may turn off some judges. That would not mean much at first but it would be pretty bad if you are trying to sell it for the future. White is one color you would want to use a lot but using it would be a bit overkill. Hence, it would be better to set it aside and let faith decide.
Also, when you are on momentum, you must keep on painting. You may wake up the next day not wanting to do the same thing you did the other day. When that happens, you may not want to do it anymore. Also, stopping a bit late would mean that you did a bit too much than what you were supposed to. Thus, better know when the best time of stopping the painting. Always look at your oil panting and ask the people around you if it is good enough. You would want to get the opinion of honest judges so that they would tell you their honest opinion about the oil painting. It is possible it needs a ton of improvement. That is one thing you must never be afraid to do.
When you have the materials, you must always do everything that can be done in order to improve it in such a way that it would come out as a better painting. If you color your dark spots with white, you may lose that sense of touch in those areas. Yes, you would want to research a bit first about the materials you are about to use to avoid mistakes.