Find the odd letters from the given alternatives.

Let's analyze the positional differences between consecutive letters in each option:
* ORU: $\text{O}(15)\stackrel{+3}{\to }\text{R}(18)\stackrel{+3}{\to }\text{U}(21)$ (Difference pattern is $+3,+3$)
* CEG: $\text{C}(3)\stackrel{+2}{\to }\text{E}(5)\stackrel{+2}{\to }\text{G}(7)$ (Difference pattern is $+2,+2$)
* IKM: $\text{I}(9)\stackrel{+2}{\to }\text{K}(11)\stackrel{+2}{\to }\text{M}(13)$ (Difference pattern is $+2,+2$)
* UWY: $\text{U}(21)\stackrel{+2}{\to }\text{W}(23)\stackrel{+2}{\to }\text{Y}(25)$ (Difference pattern is $+2,+2$)

Except for ORU, all other groups follow a uniform difference of $+2$ between consecutive positions.