-standard GO from Hummers-Offeman contains hydroxyl and epoxy species
-hydroxyl groups will be deprotonated by Calcium oxide; this might end up as a C-O-Ca-OH complex
-epoxy species might be opened in a nucleophilic way resulting in a situation similar to the hydroxy species: https://chem.libretexts.org/Bookshelves/Organic_Chemistry/Book%3A_Organic_Chemistry_with_a_Biological_Emphasis_(Soderberg)/Chapter_08%3A_Nucleophilic_substitution_reactions_I/8.6%3A_Epoxides_as_electrophiles_in_nucleophilic_substitution_reactions