Interpretation of ouput
If you use the output of your opamp as input to a hardware interrupt, you will have a lot of false triggers.  What you need is to "clean up" as much as possible, the input and output of the IR sensor.
you can start by eliminating reflections of the IR diode from triggering the IR detector.  Next, smooth the signal after the IR detector with appropriate band pass filter.  Next, use a debounce circuit to provide one TTL pulse for each position of the wheel.
There should be no need to use the opamp.
Good luck.