The difference between using ojalá with the present subjunctive or the imperfect subjunctive is the "probability" of that action to happen. Used with the present subjunctive the wish is seen as something somehow achievable while used with the imperfect subjunctive it is seen as very unlikely to happen.
1. Ojalá llueva pronto. (seen as probable)
2. Ojalá lloviera pronto. (seen as very unlikely to happen)