How to insert an OH on an aromatic ring

Here you are a typical synthetic route asked in the Organic Chemistry test, from an aromatic substrate to insert an -OH group onto the structure. 

The steps are:

1. Nitration
2. Reduction of the nitro grooup
3. formation of the Diazonium Salt (never forget the anion!!!!!)
4. finally decomposition of the diazonium salt and insertion of a molecule of water which loses the extra proton. This step free N2 Nitrogen