I can't comment particularly on subjects other than English, but by whose measure are O levels easier than GCSEs? Looking at this English O Level paper from the Guardian I would argue that actually in a lot of ways it's easier than the current English GCSE. There are more questions but a lot of them are basic information retrieval and don't really ask students to do analysis in any depth. The student is given line numbers to help them locate relevant parts of the text which they don't get in the GCSE.
Furthermore the O Level gives excerpts of texts to write about, which is the same as the current Lit GCSE, none of this 'learning quotes' nonsense that people claim they had to do for O Level. I am sure that I read up-thread someone posting that students take annotated texts into exams for GCSE? Well, not for AQA they don't (this is the most popular exam board for English in the country I think). Texts have to be completely clean and students do still have to be able to remember roughly where to go in the novel for their quotes.
The English GCSE has just been re-vamped so that coursework is all done by controlled assessment, under exam conditions, in the classroom. Unless you are very unscrupulous then you cannot cheat and the controlled assessment has meant that actually, students have largely stopped doing the very formulaic, teacher led essays that they did under the legacy syllabus.
IMO results are improving because schools are improving and teaching is improving (and we are all getting more canny about exams). You cannot be an outstanding teacher if you just teach to the test. Maybe other countries are improving faster than us but at what cost to their students and their teachers? In South Korea for example students are encouraged to work extremely long hours which, to me, does not make for a happy teenagehood.
Finally, all this is probably academic anyway. The tories have got to get their proposals past the lib dems, through parliament, past lords and past the unions. All before 2014? I doubt it.