There are only five words that use the อ sex-change doctor, all of them starting with ย.

อยุด stop   "dead boy" - sad tone
อยาก to want (to do something)   "dead boy" - sad tone
อยู to live/reside, to be at (a place); to be happening continuously (-ing)   "singing boy" - boring, no tone
อย่า don't   "dagger on boy" - sad tone

อย่าง
 

type, kind, sort, a quality of an object or person; as, like, in the way of... (changing an adjective to adverb as in -ly)

  "dagger on boy" - sad tone
อย่างไร how; whichever    

Get to know them because then for all other words spelled using อ, it can only either be:

  • a silent consonant "placeholder" for any vowel that is attached to it
  • the "awe" vowel if attached to the right side of any consonant.

The only time where there may be a potential ambiguity is when no vowel is written, as in:

อก chest invisibe "o" vowel: "ok"
อบ bake; to scent/perfume invisibe "o" vowel: "ob"
อด to go without, miss, refrain invisible "o" vowel "od"
อม hide; embezzle/cheat; suck in mouth invisible "o" vowel: "om"
องค์ organ, body part invisible "o" vowel: "ong"
องุ่น grape spacer "a" vowel: "a-ngun"
อโศก Asok spacer "a" vowel "a-soahk
อธิบาย to explain spacer "a" vowel:  "a-tibaai"

In general, except for the top five 2-letter words, which are single-syllable words, if you see อ as the first letter of a multi-syllable word and no vowel written then it is most likely to be the spacer "a" vowel.