he compared it to "english"-celtic occupation. i think that's quite fitting indeed?
Not at all. English and "Celtic" nations were in fact all Christians, though of different brands. They shared a lot of what we now call "Western" culture. Ottoman empire had a profoundly Islamic culture, which didn't mix well with Christian Orthodox cultures of Balkans and Greece.
About religions:
Religions form cultures, or cultures form religions, or both processes happen at the same time.
European culture was shaped by Christianity, originally a Middle Eastern religion adopted by majority of European nations between 0 - 1000 A.D. Most Europeans don't even realize how important Christiany was in European history, which is a pity. You don't have to be religious (I am not) to appreciate that.
One example - Magyars/Hungarians. They have a similar history as the Turks, actually. They came to Europe as nomadic invaders, did they share of killing&looting&pillaging (the usual stuff). Then they settled down, assimilated lthe remnants of Slavic population in Pannonia and a few decades later they adopted Christianity. Christianity was a "carrier" of European culture back then: with it they adopted also the latin characters, Western learning, Western values, Western social model, Western forms of government etc. It mixed with the Magyar culture and the result is today's Hungary, a Western nation with certain traits that make it different from its neigbours. Ottoman occupation of large areas of Hungary didn't change that.
Turkey had a similar history. Originally a Turkic nomads from Central Asia. They adopted Islam and with it the culture of Islamic Middle East. The post-WW1 development didn't change that, they haven't changed their cultural identity. They tried to, but it was impossible. To uproot 1000 years of their unique cultural and social evolution and replace it with culture which most Turks found "alien".
Today, Turkey is the most advanced (economically, socially, politically) country of those which share the culture build upon Islam. It is not an European country though.
So, to close this: it's much harder to "convert" or "assimilate" a whole country to a different culture. You can assimilate individuals though.
Just for the record, what is 'European culture'? Does it exist?
AFAIK there isn't enough in common between EU Nations to call it 'culture'.
AFAIK you're wrong, and I am not gonna explain it all over again. Google it, or use the search engine on this forum.