Italianjon, if you are flying VFR, then the semi-circular rule is only a recommendation, and the safe altitude marked on the chart (grid MORA; MSA is minimum sector altitude, a totally different animal) will only apply if you are not in sight of the surface, otherwise you can route around the highest points.
Regarding the oxygen requirements when flying non-commercially, you are better off checking your own tolerance to hypoxia - either use a pulse oxymeter as Bookworm recommends, or get yourself thoroughly checked in a hypobaric chamber - I know there is one at the
Institute of Aviation Medicine in Prague, and probably a handful more of them around Europe. From my prior mountaineering experience, skinny people are usually more tolerant to hypoxia than fat ones. Also, smoking even a single cigarette reduces your tolerance for up to a day afterwards.