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

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 (usually written informally or spoken as ยังไง)    

Get to know them because then for all other words spelled using , 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.

When a vowel is attached to it then it just becomes the sound of the vowel. The อ letter itself is silent (actually, it's the "glottal stop" so that you can launch into the vowel sound):

อา โอ ฮี อึ เอา

When it's attached to the right side of a consonant then it is the "awe" vowel:

รอ บ่อ นอน

The only time it can be potentially confusing is when it's a two-letter word with the invisible or implied "o" vowel in-between; or when it's fused together with another consonant, making it unpronounceable as it stands:

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 suck, keep in mouth; hide; embezzle/cheat invisible "o" vowel: "om"
องค์ organ, body part invisible "o" vowel: "ong"
อร่อย delicious spacer "a" vowel: "a-roi"
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.