very abstract question but I'm just collecting information for now. Can subsidiaries from US and Europe exists in OneWorld account without multibook feature? I know almost nothing about US accounting standards but I'm pretty sure they could be quite different from European ones. Anybody has experience in such case?

Absolutely - the client I'm just transitioning off has US / Germany / UK without multibook.

You'd use multibooks if you want to have different accounting treatment for the same subsidiary. But in case of separate subs in different countries there is no need for multi-books.