Is it permissible to supplicate for the deceased with: "O Allah, forgive my beloved so-and-so who has passed away; O Allah, forgive him and have mercy on him"?

There is no harm in supplicating for the deceased with: "O Allah, forgive him, have mercy on him, grant him well-being, pardon him, honor his reception, widen his entrance, wash him with water, snow, and hail, and purify him from sins as a white garment is purified from dirt." It is permissible to supplicate for the deceased with any formula that conveys mercy and seeking forgiveness, but it is best to supplicate with the narrated (ma'thur) prayers. "O Allah, forgive him and have mercy on him" is part of a supplication narrated from the Prophet, peace and blessings be upon him.