There are three wires to/from the O2 sensor. The two that are the same gauge and use the same connector are the power and ground for the sensor. The single green (?) wire with a different plug is the signal wire to the ECU.
You won`t damage it by reading the voltage from the sensor. Leave the power/ground wires connected to keep it properly heated and make sure the motor is good and hot.
The green signal wire is shielded, so be careful if/when you decide to splice into it.