You might try a degreaser. You can find them at big box hardware stores like Home Depot. It comes in a 1 gallon concentrate that you can dilute to your needs.

The other option is one of the traditional oven cleaners.

The splattered grease is harder to remove once the oven cools and the grease hardens. If you wipe it with a paper towel as you are cleaning up the dishes, the grease should still be soft enough to come off easily.

Use a good strong spray cleaner like "Cinch" or "Formula 409" or "Fantastic" and spray when the oven is somewhat cooled off. Better to do a little cleaning every day than to have to tackle a masive job less frequently. Use paper towels.

If the oven has hard baked on food/grease, try putting a bowl of ammonia inside and leave overnight. Next day clean out the softened grime.

I would do this outside in order to keep the strong fumes out of the house.

After cleaning, always use a damp paper towel to clean out splatters after each use. A quick wipe every meal used will prevent or extend time before next major cleaning.

Aluminum foil works great on drip pan (change often), and maybe other areas of the oven in order to reduce cleaning splatters. Use this cautiously, sometimes aluminum foil can become fused onto things if left on too long.

An ounce of prevention is worth a lot!