Most Achronim agree that oilam haba refers to yemois Hamoshiach where there will be a guf and a neshama. When a person dies he/she goes to oilam haneshomois, kind of like a storage facility until techiyas hameisim, where there will be oilam haba a real life and geheinoim also real life. If you were wondering how a person can burn in gehinoim if he dies, the gehinoim kicks in after techiyas hameisim
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