Syllables ending with the vowel o, like o お, ko こ, go ご, so そ, etc; and precedes the vowel う alone will make the お vowel be pronounced twice longer, the same will happen with syllable ending with え+い which will make a twice longer え sound. For example, がっこう (school) and せんせい (teacher).
Oh, something that might raise questions, when you see a smal tsu like that ( っ ), it means the next syllable will have a double consonant. The word school I used as an example last comment has this small tsu, because it's pronounced gakkou. I don't know if there are any other usages for such character.
Hope I helped.
It lengthens the vowel sound. If you want the long answer for this word in particular, words from Middle Chinese were imported into Japanese, but the Japanese phonological system changed the -ng ending on Chinese pronunciations to a long vowel, so you have sei instead of seng for 生, gyou, gou or kou for 行 from Chinese hang (the Japanese was originally a instead of o but evolved), and so on.
Wwll, I noticed that I frequently know all of the other words, so the guessing is easy. It's okay anyway- guessing wrong is a fine way to learn. In language learning it works best to relax about mistakes. - Not that I'm a great example. I know full well I need to speak to other people more in the target language, but I'm afraid of making a fool of myself. But that is the only way to learn!