From what I understand, the hiragana characters above should make the word jidoushiya, but for some reason it is actually jidōsha.
Why did the word change this way even though the characters are:
じ → ji, ど → do, う → u,し → shi and ゃ → ya?
The small ゃ is different to the larger や. When using the smaller ゃ after a character in the i-line, it modifies the sound before it;
So, しや='shiya' but しゃ='sha' Similarly, りや='riya' but りゃ='rya'
As for the ō, this is a notation which refers to an extended sound. Following お with う extends the sound to be twice as long, and this can be written either as 'ou' or 'ō' depending on personal preference.
You can also do similar things with katakana such as ティ='ti' because this isn't normally a sound you can make in Japanese. These can usually be guessed, though.
The combined form is said as 拗音 (yō-on), as contracted form which special mora (syllable) formed by palatalized sound. Unlike chō-on (長音 = long sound) which counts as 2 moras, even written with 2 letters (the second one is smaller form of ya, yu or yo) it considered as single mora.
自動車 (jidōsha) as example word consists of 4 moras, all of them are on-readings from their respective kanji:
NB: Katakana form "ティ" (ti) doesn't considered as yō-on even using similar construction, it is part of additional letters specially created to form English or foreign loanwords/gairaigo (外来語).