You don't need the op-amp and it will cause more trouble than it's worth! You just need two resistors to make a Voltage Divider so you can read above 5V.
With the default 5V ADC reference only need a 3:1 or 4:1 voltage divider. You may want some "headroom" so about 4:1 might be better...
The default ADC reference is the chip's supply voltage (Vcc). i.e. If Vcc drops, your voltage reading/calculation will increase.
If you are using the onboard 5V regulator Vcc should be fairly stable and "close" to 5V (depending on how much accuracy you need). If you are bypassing the regulator by using USB power or using batteries, or an external supply (or feeding it too low of a voltage into the regulator) your reference can vary and your readings will vary.
The optional-internal 1.1V is super-stable but it has a tolerance (I think 10%) so you may have to calibrate your code. And of course you'll need a different voltage divider resistors since the Arduino won't be able to read above ~1V.