In Evil, Ctrl-o is not working like in Vim

[Ed Kostas]

When I press Ctrl-o in insert state, then press :w, Evil remains in normal
state after executing the colon command.  

When I press Ctrl-o in Vim, then the :w command, the command is 
executed and Vim returns to the insert state.

In other words, in Vim, Ctrl-o executes one command and remains
in insert state. In Evil, it remains in Normal state after executing the 
command.  

It would be fine if Ctrl-o had exactly the same behaviour in Evil and
in Vim. Is there any reason for this not being so?