Monthly payments. At the beginning of a mortgage, I-O and option-ARM payments are likely to be lower than traditional mortgage payments. But when the I-O payment period ends or when your payment-option ARM loan is recast, your payments could change a lot. If you have a 30-year mortgage with a 5-year I-O payment, you will have only 25 years, instead of 30, to repay the principal, and your monthly payment will rise. With a 30-year payment-option ARM, at the end of the first 5-year period, your loan is recalculated based on a 25-year term. In some cases your monthly payment could double or even triple.