The two most used pdf annotations, text cross-out and text replace, aren't present in Okular. To compare, check foxit-reader for linux that seems to implement the full set of annotations
You can configure available annotations in Okular (Settings->Configure Okular->Annotations). The cross-out should be available (add a 'Text markup' annotation tool with Type 'Strike out'). Replace seems not to be available (at least here, Okular 0.23, poppler 0.24.5).
Given that cross-out is available as mentioned by Jonathan, this is a duplicate of Bug 332835. *** This bug has been marked as a duplicate of bug 332835 ***