I guess this refers to the pronunciation of the 'spoken' odd letters ie. ay, cee, ee, gee as opposed to the even letters bee, dee. ef etc. If that's the logic then I will guess at 'o' ?Ok.. This I just found out about this last week and I was very surprised...
What vowel is always odd?

they used a sundial during daylight hours and a Greek water clock during the night. But how were the hours themselves different ?