When I did my GCSEs in 1993 (getting old now... :( ) the term O-Levels was still fairly common- mainly used by those who had done them, or who used to teach them.
The system then was that one took GCE exams, i.e. a General Certificate of Education, which were divided into Ordinary (O-) levels and Advanced (A-) levels. The GCSEs, which superseded the O-levels, were an abbreviation of General Certificate of SECONDARY Education, although the term A-levels was left intact.
Therefore I would suggest flagging O-levels as (obs.) and adding GCSEs as a translation for Mittlere Reife.
As far as the original suggestion is concerned, that is supported- it should indeed be O-levels with a plural "s"- unlike in the German system, in England it is possible to pass or fail all of your subjects individually, thus it would be possible to have only passed one GCSE or O-level. However, that would hardly equate to the "Mittlere Reife". The norm, when I did my GCSEs was to take 10 of them, and generally speaking I think the necessary minimum requirement to be allowed to do A-levels was that one had to have passed at least 5 GCSEs.
On another note, neither the Scottish nor Irish Abitur equivalents seem to be listed yet.