[*]The operation works on Y data, but uses the X information.
The X domain of the first operand defines the limits of the calculation.
[*]This is an assignment operator.
The operation is performed using the second dataset and the results are put in the first dataset.
So if you need to preserve the first dataset, you should copy it and use the copy. Both the X and Y data must be copied.
[*]The operation uses interpolation.
Since a particular X may not exist in the second operand, Origin will interpolate a Y value for the calculation. If the datasets are NOT plotted, then linear interpolation is used. If the datasets ARE plotted as Line or Line+Symbol, then Origin will interpolate according to the connection method used. This can be particularly useful for the Spline connection for example.

data1_b +-O data2_b; // Add Data2_B to Data1_B
data1_b --O data2_b; // Subtract Data2_B from Data1_B
data1_b *-O data2_b; // Multiply Data1_B by Data2_B
data1_b /-O data2_b; // Divide Data1_B by Data2_B