I'm trying to teach myself to read Thai, but I'm not in Thailand and have no one here to ask.

 

When there is no vowel written between consonants, sometimes  an oh sound is pronounced, but sometimes an ah sound, for example นก and ทหาร. How do you know when it's oh or ah?

In single syllable words, it's /o/.

 

In two syllable words, an unwritten first vowel will usually be /a/ (unstressed, no glottal stop, mid tone).  Any subsequent unwritten vowel will probably be /o/.  So, for example,  ขนม is khaˑnǒm.

 

However, where are exceptions, such as นวนคร, which is pronounced naˑwaˑnaˑkhɔɔn.

 

However, there are other possibilities (as is shown by คร in นวนคร and by บริกร - bɔɔˑríˑkɔɔn).

 

I'd suggest you get yourself a good reference work to cover all the possibilities.  (And even then there will be some irregularities left.)

This gives a good description of all the various contructs (including the two raised by the OP)

http://thai-language.com/id/830221
http://thai-language.com/id/830222

 

.. but, yes, you will need a good book to cover everything.  I like Teach Yourself Thai by David Smyth.  Also the FULL Linguaphone course (not the single book and CD version) is good, (but expensive).

 

one of the many rules to get used to.  will take a long time, if you stop and ask here everytime :)  ;  I believe Mr.Smyth  also has a grammer book for more depth  and/or Mr. Higbie

I'm trying to teach myself to read Thai, but I'm not in Thailand and have no one here to ask.
 
When there is no vowel written between consonants, sometimes  an oh sound is pronounced, but sometimes an ah sound, for example นก and ทหาร. How do you know when it's oh or ah?

Do you have a dictionary bookmarked? If not find Longdo dict. You can learn to read from it and it will tell you all you need to know I think, much more than all the grammar books written in English.
eg. Type in ทหาร and scroll down to this: พจนานุกรม ฉบับราชบัณฑิตยสถาน พ.ศ. ๒๕๔๒ and you will see ทหาร น. [ทะหาน] followed by a definition.
Do try it and come back if you have any questions on syntax definitions etc.


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Thanks for the feedback and recommendations everybody! You've all been quite helpful.

--Tom