As a first step, place a resistor between the Teensy A14 pin and the rest of the world. If something horrible happens on the other side the resistor, the voltage will mostly go across the resistor. With a little luck, the resistor will limit the current which flows into Teensy to a safe level.
A value like 4.7K to 10K provides a lot of protection, but even lower values like 1K or 470 ohms will help.
If you want to do even more, use 2 resistors in series, and places a zener diode between their junction and ground. The cathode (side with the stripe) connects to the resistors and the anode connects to ground. Use a 3.6V or 3.9V zener. Then if something horrible happens, most of the voltage will be between the opamp and the zener, with most of the current that would have flowed into Teensy being diverted to GND through the zener. Depending on the current, the zener might go to 4 or even close to 5 volts, so you still need the resistor between the zener and Teensy.
Of course, the trouble is this adds resistance between Teensy and your opamp. Maybe that's a non-issue, if you're running the signal directly to the positive opamp input, and if your opamp's input bias current is low, or if you don't care about the tiny extra error due to the opamp's bias current flowing through that resistor. If you do care, often a similar resistor can be added somehow on the negative side of the opamp, which causes a similar voltage error on the other side, so they mostly cancel each other. As will all analog circuit design, small details can range from being insignificant to contributing terrible errors.