The International Bac program is, in my opinion, the bar which most national-level secondary educational systems should meet or exceed (sadly, the US is well behind here). If I recall correctly, O-levels are a lot less complicated (as well as being less advanced) than A-levels.
In New York state, we had four core subjects -- English, Social Studies, Math, and Science -- for which our Board of Regents mandated a minimum level of study (4 years English, 3 years Social Studies, 1-2 years Math -- through Elementary Algebra -- and 2 years Science), plus a three-year sequence in one of the following: Math, Science, Modern Language (choose one and stay with it for three years), Home Economics, Art, Industrial Arts, or Music, plus four years of Physical Education. Some high schools had higher requirements -- mine required 4 years of Social Studies. The only mandated exams for a local high school diploma are the school's own exams plus the state's "minimum competency" exams.
Most colleges and universities require certain other standardized exams for candidates for admission. The standards are the SAT (School Admissions Test, I think?), ACT (don't remember what that one stands for), and Achievement Tests (administered by the same group that administers the SAT).