Which of the following has higher ${\mathrm{IP}}_1$ ?

Electronic configuration of Oxygen $=\left[\mathrm{He}\right]2{\mathrm{s}}^{2}2{\mathrm{p}}^4$

Electronic configuration of Sulphur $=\left[\mathrm{Ne}\right]3{\mathrm{s}}^{2}3{\mathrm{p}}^{4}3{\mathrm{d}}^0$

Electronic configuration of Selenium $=\left[\mathrm{Kr}\right]4{\mathrm{s}}^{2}4{\mathrm{p}}^{4}4{\mathrm{d}}^0$

Electronic configuration of Tellurium $=\left[\mathrm{Xe}\right]5{\mathrm{s}}^{2}5{\mathrm{p}}^{4}6{\mathrm{d}}^0$

Ionisation potential decreases with increase in the number of shells since it becomes easier to lose an electron due to less attraction between nucleus and outermost electron.

Thus, oxygen has higher ${\mathrm{IP}}_1$.