Quote from peteroupc »When an effect has one object "enter the battlefield as a copy" of another object, the first object enters the battlefield with the second object's copiable values (C.R. 706.2). These copiable values, however, don't include effects (including effects of the form "becomes a [characteristic]" as found in Polymorphist's Jest and Oko's second ability), with one exception being copy effects (C.R. 706.2). In this respect, compare Oko's second ability and Polymorphist's Jest with Mirrorweave, which says "becomes a copy...", which does express a copy effect.
Note also that if an artifact that's normally an artifact becomes a creature due to Oko's second ability, it will no longer be an artifact (C.R. 205.1a; compare that ability with Tezzeret the Seeker's last ability, which says "Artifacts you control become artifact creatures...", which expresses an effect that does leave the permanents' prior card types untouched [C.R. 205.1b]), but even so, the artifact type will still be part of the permanent's copiable values (C.R. 706.2; see also C.R. 613.1, 613.1a), which copy effects such as Clone's can generally access.