Will the insert fog as badly as my glasses do under OTG goggles?

No—this is the primary advantage of Rx insert goggles. The insert sits inside the goggle's sealed, dual-lens thermal barrier and benefits from the same anti-fog coatings and ventilation as the outer lens. Your glasses under OTG goggles are exposed to warm face air on one side and cold goggle air on the other, creating the fogging problem. The insert is fully enclosed in the goggle's climate-controlled environment, dramatically reducing fog risk. However, the insert does reduce internal air volume, so premium ventilation is still important.