What else can I take if peppermint oil isn't relieving my nausea?

Several things. Try ginger root. You can use it in stir frys. Also fennel root and fennel seeds are good as is oregano.
Ginger. Ginger is the best herb for nausea in tea, or soup, or grated on food. If the nausea is due to pregnancy, it may help to eat frequent small meals. Low blood pressure can cause nausea in which case avoiding dehydration might help by drinking coconut water or a rehydrating solution like pediatlyte.
Ginger. Ginger is an ancient tonic used for nausea in chinese medicine. To make ginger tea: pour boiling water over freshly sliced, organic ginger and steep for five minutes in a covered pot. Allow to cool and then sip. One can also nibble on chrystalized ginger or sip on slightly flattened gingerale.