The /o/ vowel sound is generally not written: it is implicit when a syllable starts either with a consonant or
a consonant cluster, and ends with a consonant. So, for example, /phǒm/ (meaning "I") is written ผม.
/troŋ/ (straight) is written ตรง. As usual, the tone is determined by the initial consonant, and any tone mark
is written over the first consonant (or the second consonant if it's a consonant cluster).
Be aware that some words that might look as if they start with a consonant cluster don't, and
have ว to provide the /ua/ sound, e.g. กวน (to disturb) is pronounced /kuan/ not /kwon/,
and ขวด (bottle) is pronounced /khùat/, not /khwot/.
Here are some more words, some of which have ว acting like this.