Why does the output of this opamp change my analog signal?
OP27 is not a rail-to-rail input op-amp. Here's the relevant section of the datasheet:



Notice the input range is specified with +/- 15 V supplies. This means the input voltage must be (for a typical chip) at least 2.7 V above the negative rail and 2.7 V below the positive rail for the op-amp to work as expected. This is obviously impossible to achieve with a 5 V single supply.

You'll need to increase your supply voltages, and be sure to bias your input within the recommended operating range. As mentioned in comments, you will also need to have a negative supply if you want the output voltage to be able to reach below ground.

Or choose a more modern op-amp designed to work with lower supply voltage.