If you qualify to apply for an OIC, the IRS will then determine how much it will accept from you to pay off the debt. The amount of the offer is also called reasonable charging potential (RCP). This is the amount that the IRS can reasonably charge you before the collection statute expires. Determining the optimal amount to offer to the IRS isn't easy.
It takes a lot of experience to know what the sweet spot is for a given case. However, in general, you can start with an estimate of your disposable income for a year and add to that any valuable assets that you can sell for additional cash. The IRS is primarily concerned with ensuring that you receive at least the same amount of your reasonable collection potential, so make sure that the amount of the offer is higher than your RCP in any case.