There are a lot of factors that determine the difficulty level of oil painting.

Experience level

Everyone starts out a novice. If you've never do much a doodled on a notepad or held a paintbrush, oil painting can be a bit overwhelming. Having experience with other mediums can be very useful- so if you're adept with pencils or you've dabbled in acrylics, many aspects of oils will be easier for you (depending on your style of painting).

Practice

Your level of devotion to learning a new craft will determine a lot- if it's something you passionately pursue, putting lots of time and effort into it, you'll likely find it getting easier much faster than someone who only breaks out the brushes once a week. Doing daily sketches, small challenges to teach yourself and keep your skills sharp will go a long way to make painting “click” for you- trust me.

Mentoring

Having someone to learn from is not essential, but can definitely make learning easier. Having a good teacher who can guide and explain while allowing you to work in and develop your own style is great if possible. However if this is not a good option for you, don't worry- many a wonderful artist has learned entirely on their own.

Natural Skill

This is a tricky one. Some people are innately artistic, and oil painting will come more easily to some than others. This does NOT mean that it can't or shouldn't be tried by those who don't have a natural flair. Innate talent only takes you so far without practice and experience.

Also, there is something to be said for the intention of the artist- if you're picking up oil painting with the desire to paint larger than life realistic pieces, it will likely be very daunting. However if you want to create small expressionist pieces as a way to channel emotion, that could be far less difficult.

So there can be a pretty wide range of answers to how easy it is to oil paint- if you have experience drawing, time, passion and innate skill, it may be pretty easy to start— though for most of us life doesn't make it that easy. If you haven't touched paint since 6th grade art class and have never felt an affinity for it, it will be difficult- but could be very worth it.

The most important factor is practice. The more you do, the better you become, the easier it is.

A lovely quote by Michelangelo helps keep me focused on the pursuit of frequent practice and improvement:

“If people knew how hard I worked to get my mastery, it wouldn't seem so wonderful at all.”