OpAmp Spice Model: Offset vs Common Mode Voltage
You are using bipolar supply for OPAx350 High-Speed, Single-Supply, Rail-to-Rail Operational Amplifiers MicroAmplifier Series. If you change V1 to be 5 and V3 to be 0, you'll see a different picture. If you want to change to offset voltage, you need to dig into the model, as suggested. For that you'll need much more than simply change a .param, as the model is quite involved. You could start at the VOS_DRIFT_OPA350 subcircuit, then see how it fits in the rest of the netlist (I see XV_OS in the main subcircuit), but I don't feel particularly interested in unravelling the rest. If you wish to do so, by all means.