O Levels are the old name for qualifications now known as GCSE's (General Certificate of Secondary Education). They are as has been said normally taken in the last year of secondary school (in UK age 16). The grades are A* which is the highest followed by A,B,C and in my day were considered to be a pass (although we didn't have A*) the following grades although awarded were (in my day) generally deemed to have a fail grade D,E,F G - you can also get a U which means that the exam is Unclassified. A Levels are taken after GCSE's and are normally taken at a 6th Form College, these are done in two separate stages AS level which is completed over 1 year and then you can proceed on to the A2 year which will then give you a full A Level, these are graded A - E with A being the highest grade.