No worries, you can pre-book your baggage allowance now for 20 kg or 30 kg and save up to 50% of baggage charges paid at the airport. 6 OMR for 20kg (vs 12 OMR at the airport) and 8 OMR for 30 kg (vs 16 OMR at the airport).
The combined weight of all pieces must not be more than your pre-purchased checked baggage allowance. No single piece should be larger than 75 cm x 55 cm x 35 cm or weigh more than 32 kg.
Provided there's space in the hold for them, extra weight or pieces of luggage will be charged at a per kilo weight, even if the total weight for all pieces is less than the allowance you purchased.