The pronunciation tab there is paywalled, but in the OED app on my phone, they list /əˌsəʊsiˈeɪʃn/, followed by /əˌsəʊʃiˈeɪʃn/ in the pronunciation section. I honestly can't recall ever hearing a single person in the US pronounce it /əˌsəʊʃiˈeɪʃn/, and though I won't say nobody does, the other pronunciation is far and away more common, where the 'c' and 't' make entirely distinct sounds.