Why? In Sino-Korean numbers*, 오/五/o means five. So sometimes the number five reminds me of this song.
There is also sort of a cheer-leading concept in the song. SNSD will cheer for you until the results come out.
5-12 days now... Ah! Now 5-12 reminds me of the 5-12-13 Pythagorean triad too.
*In Korean, there's 2 main sets of numerals; native Korean and Sino-Korean.