A full dive medical is really only necessary for working divers--instructors, commercial divers, etc. It may include respirometry tests, hearing tests, exercise tolerance tests, chest X-rays and more.
Provided you have no preexisting medical conditions which could increase the risks to you of SCUBA diving, a normal GP can sign you off as fit to dive, if a doctor's signature is required by local legislation (for most countries, self-certification via the diver medical form is sufficient). However, if you do have any of these conditions (i.e. you have to write 'yes' against any of the medical declaration form questions), you should get the sign-off from a doctor qualified/experienced in diving medicine/physiology--which your GP likely will not be. Depending on the nature of your condition(s), the dive-doc may require you to complete some/all tests from the full dive medical before declaring you fit to dive (or not, in some cases).
Assuming you get signed off as fit, the PADI OWD course is not difficult to pass, as c-kayak says. The course is designed to make learning to dive as easy and inclusive as possible. As you might expect, you learn the basic dive skills/ knowledge first, then build on those later. The course is performance-based, which means that the instructor is required to ensure each student's mastery of each skill/theoretical concept (to a specified standard) at each stage of the learning process, before introducing the more complex skills/knowledge. Once you complete all the portions of the course (including the open water dives), you get your certification (NB not a 'license'--it holds no legal status).
There are two major factors that determine the outcome of a course--you, and your instructor. As a student, you will be informed (by the manual, videos and your instructor) of what you are required to learn at each stage. Nothing in the course is particularly challenging for a student of average fitness and mental ability, provided they are comfortable in the water, and apply themselves (RTFM, or at least WatchTFVideos...).
If a student has a genuine difficulty with anything (most often as a result of a psychological block, rather than anything else, in my experience), asking for help is always the best option. Which brings us to the other factor, the instructor. A good instructor will give each student the time and practice they need to work on their weak points--and a good dive centre manager will recognise that not all students will get everything on their first go. An instructor who rushes his/her students through the course is doing them no favours,in my opinion, and may indeed be breaking the spirit (if not the letter) of the standards.
Find an instructor who loves diving, who inspires trust, and who treats you as a person, rather than a course fee. All the best.