I am currently an FOO for an airline and have ended up with the task of writing the training manual for the Ops department based on the new FOO/FD syllabus, primarily due to my having gotten my frozen ATPL and so being familiar with both the flying theory and the FOO/FD role.
The ICAO syllabus in my opinion is VERY OTT. how many FOO's ever need to know about D, E and F layers of the ionosphere for instance or the formation of the ICAO body. There is an awful lot of niff naff and trivia in there and so I have ended up using the syllabus purely as a guide and missing out the stuff deemed by myself and my manager to be irrelevant to the role of FOO/FD in our company.
If you were to stick to the ICAO syllabus and know the subject in the kind of detail it seems to require then you would probably be just as well to go and do your ATPL course at the same time!!
Hope that helps
Boxjockey