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?
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2016-02-24T12:04:14.517
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2016-02-25T04:49:29.507
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pronunciation
Why is う sometimes read as "o" instead of "u"?
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).
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2016-02-25T03:34:24.090
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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".
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2016-02-25T04:28:26.233
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