pj123

A non immigrant O visa is considered category "Other".
It is simply means a "doesn't fit into another category" type visa.
If you had intended to work while in Thailand then you ideally shhould have applied for a non immigrant B visa....a Business visa which would have allowed you to be employed.
But to get that non immigrant B visa would have required an offer of employment from a Thai company.
However, the good news is that the non immigrant O visa you have will allow you entry into Thailand with no problem.
Since you intend to work, you will need a job, The Thai company who offers you that job will initiate the paperwork to get you a Work Permit. With that work permit your visa will also be changed from a non immigrant O to  a non immigrant B which allows employment.
That work permit, and the change of visa type can be done in Thailand.
The thing you need to understand however, is that a non immigrant type B visa depends on you having that work permit and that job.
If you lose or quit that job, you lose that type B visa and must leave the country then.
All that starts with your Thai company making you a job offer and initiating the paperwork.
A visa, irregardless of type, gets you into country, then you can convert it to one on which employment is allowed here.
It's the work permit...and the job offered by a Thai company...that is the key to the whole process.
At least that's the way the process is supposed to work.