Create the oxygen atom

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