By definition "Flexi Task" action in on premise gives you three things:

1. Possibility for delegation;
2. Functionality to approve/ reject the task using response in the email, or using response in Lync/ Skype for Business;
3. Be notified via the SMS.

Of course there is much more, like the option to send notifications only for a selected set of approvers or not send them at all, etc...

In Nintex for O365 "assign task" actions allows you only to respond for the notification email so that you can approve or reject the task without visiting SharePoint at all. But again with limitations - there is a set collection of possible responses and you cannot define your own like you can in on premise.

Other functions like SMS/ Lync/ Delegation etc... are not present. For some you will find workarounds, some are just impossible.

Moreover in Nintex O365 you have two assign task actions, that are nearly the same with one, major difference:

1. Assign a task - allows you to assign a task to a single, particular user and in return gives you the ID of a created task.
2. Start a task process - allows you to assign a task to multiple users/ groups and set behavior accordingly (wait for all responses or only for the first one, assign a task to everyone at once or sequentially) and it does not give you ID of created tasks in return

Summarizing - there are no "Flexi Tasks" in O365 as we all know them from on premises 

Regards,

Tomasz

Ah okay well that has cleared it up thank you very much