Of course it is impossible to dissociate oxides. Oxides do not dissociate into ions! They can only dissolve in solvent (e.g. water) as a whole particle. In this exercise there is a reaction between water and MgO:
So, we now know that from 40 g of MgO, there is a creation of 58 g of Mg(OH)2. So, from 0.0086 g, it will be created 0.0125 g Mg(OH)2. We assume that this amount is so low that do not influence the calculations concerning solution volume. Therefore, we assume that 0.0125 g of magnesium hydroxide is in 0.1 dm3 of solution (100 g of water occupy 100 cm3 in normal conditions). Thus, the molar concentration of Mg(OH)2 is equal to 0.0022 M. From 1 mole of magnesium hydroxide we get 2 moles of hydroxides ions. In this situation we obtain 0.0044 M of hydroxide ions concentration. pOH is equal to -log(OH). pH is equal to 14-pOH.