Add the hydroxyl group

Select the Structures → Ligands → OH structure
Double-click on one of the hydrogen atoms

Again, the hydrogen is replaced by the structure. In this case, the oxygen replaces the double-clicked atom. Notice that the hydrogen is precisely aligned along the C-O bond and points away from the rest of the molecule. This shows you the very general way in which the structures will align according to the bonds in the original molecule and those in the structure. In this case, the hydroxyl group is not immediately orientated as it normally would be in an ethanol molecule:

Click the preoptimize button