Why is う sometimes read as "o" instead of "u"?

Question

As an example, きょう is read as "kyoo", despite the last kana being usually read as "u".

Why and when does this happen?

Answer 1

In most cases う after お、こ、そ、と、の、ほ、も、よ、ろ makes "o" sound longer.

For example: こうえん（kōen)、きょう (kyō)

There are some cases in which お is used to make "o" longer.

For example: おおきい (ōkii)、とおい (tōi)

There are also cases in which う is a suffix and read as う。

For example: おもう（omou、[思う]{おもう})

Learn more about long vowels.

P.S.: ō is for long "o", same as "oo".

Answer 2

You can read おう as おお beside おう for the purpose of convenience of pronunciation unless it's combination of お included in a verb stem and suffix う (e.g. おう as in 追う、おもう as in 思う etc.), or it bridges components of a word compound (e.g. いご-うち: 囲碁打ち).