Choose the letter pair which is different from others in the group.

Let's find the standard alphabetical positions of the letters in each pair and calculate their difference:
* GB: $\text{G}=7$, $\text{B}=2⟹7-2=5$
* TO: $\text{T}=20$, $\text{O}=15⟹20-15=5$
* OK: $\text{O}=15$, $\text{K}=11⟹15-11=4$
* MH: $\text{M}=13$, $\text{H}=8⟹13-8=5$

All pairs have a positional difference of $5$, except for OK, which has a difference of $4$. Therefore, OK is the odd pair.