Answer:-
[tex]\boxed{\sf \sqrt{x^2+y^2}}[/tex]
Explanation:-
We know distance formula
[tex]\boxed{\sf \sqrt{(x_2-x_1)^2+(y_2-y_1)^2}}[/tex]
[tex]\\ \sf\longmapsto OP=\sqrt{(0-x)^2+(0-y)^2}[/tex]
[tex]\\ \sf\longmapsto OP=\sqrt{(-x)^2+(-y)^2}[/tex]
[tex]\\ \sf\longmapsto OP=\sqrt{x^2+y^2}[/tex]