O class is a business saver fare (and business ‘Flex awards’ also book in O so yes, you are correct). The availability of F awards / upgrades (booking in Z bucket) does not depend on what the original fare bucket in business class was. That upgrade does however require more miles than an upgrade from a Flex or Flex+ fare.
To answer the question: it is difficult to predict the likelihood of upgrades becoming available. It certainly depends on the route, but obviously also on other factors such as (predicted load).