If someone has pain in his face or arm and is obliged to use oil or cream on them all the time, what can he do to perform the prayer? What is the ruling of this case on fasting ?

All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad is His slave and Messenger.

In principle, if such oil or cream constitutes a thin layer that prevents water from an organ (hand, leg…) of one who is performing ablution as required, then one's ablution becomes invalid (i.e. one has to remove it before ablution). There is no harm if it is applied after ablution.

At the same time, there is no problem if such oil or cream does not constitute a layer [that prevents water reaching the skin].
If one who uses this cream is a patient and such cream is definitely needed and using water spoils it and delays recovery, then one can wipe over it and complete his ablution. But if wiping may spoil the cream, then one can bandage the place (according to need) and wipe over the bandage and complete his ablution.

Furthermore, if such wiping may also spoil the cream, one can perform ablution without touching the spot that is covered with cream, and then, perform Tayammum. If one cannot perform Tayammum, then he can perform ablution without washing the spot covered with cream.

Allah Says (what means): {So fear Allah as much as you are able.} [Quran 64:16]

Applying cream does not affect one's fasting.

For more benefit on the nullifiers of ablution, please refer to Fatwa 84093.

For more benefit on the detailed description of ablution, please refer to Fataawa 82845 and 83164.

For more benefit on obligatory and Sunnah acts of ablution and prayer, please refer to Fatwa 84841.

Allah knows best.