I've entered Thailand 6 august and received the normal 90 days stamp. My present 'O' non-imm visa runs out Sept 4. Am I allowed to have another 90 days without hassles if I go to the border Sept 3 ?

Yes you are providing you have a Multi Entry Visa

If you have a multiple entry visa you will get another 90 day entry without a problem.

As it is now you have permission to stay expiring around November the 2nd ??

 

Make sure your visa is more than a single entry visa or when you re-enter Thailand you'll just get a 15 or 30 day permission to stay ending on or about 18 September or 3 October.

 

If you do have another entry on the visa, you'll get a permission to 2nd December or thereabouts.

Thanks.  I have a multiple entry Non-Imm 'O' so it seems I am able to stay completely legally until early december which will fit into my plans perfectly.