Heat oil in a large skillet. Brown oxtails on all sides. Remove oxtails to a plate and add carrots and onions to the skillet; cook 10 minutes until soft. Add paprika, salt, saffron and pepper; stir well.
Place 1/4 of vegetables in a 4-quart crockpot. Add oxtails in a single layer. Repeat layers and continue until vegetables and oxtails are all used up. Add any juices left in the plate plus the 1/4 cup water.