Can one give O-level and A-level examination in session?

No, you cannot generally take O-Level and A-Level examinations in the same session. O-Levels (or GCSEs, their equivalent) are typically taken at a younger age than A-Levels. The A-Level curriculum builds upon the knowledge and skills gained at the O-Level/GCSE stage. Therefore, it's structured sequentially; you'd complete O-Levels/GCSEs before starting and subsequently taking A-Levels.