To create the oxygen atom you need to switch to the O-tool:
Select the O-tool by clicking on the button with the 'O'
With the O-tool, create an oxygen atom bonded to the second carbon;
Click somewhere in the drawing area
The oxygen atom has been added.
For now, we are done using atom tools, so go back to the select tool:
Select the select-tool by clicking on the button with the arrow
