I stayed in Thailand for more than 10 years on yearly visa extensions and I visited my home country Belgium every year for several weeks and enter Th again with a re entry permit . Due to medical treatment now I stayed in my home country for 6 months so I could not extend my existing visa and obtain a re entry visa . The immigration office gave me the advise to apply for a new non immigrant O visa 90 days in my country and then extend in my local Th immigration office . I fulfill al the requirements but I have one question :

When I apply for a non immigrant O visa on line , is it sufficient to buy or reserve a one way ticket or do I have to buy or reserve a return ticket .

One way sufficient

Only need a return ticket when entering VE, or TV, not a Non Immigrant type visa.

19 minutes ago, Liquorice said:

Only need a return ticket when entering VE, or TV, not a Non Immigrant type visa.

For Tourist visa suggest that even if e-Visa application asks for exit date proof of actual onward ticket not required.

Also not required by airline at departure flight to Thailand

1 hour ago, DrJack54 said:

For Tourist visa suggest that even if e-Visa application asks for exit date proof of actual onward ticket not required.

Also not required by airline at departure flight to Thailand

Others would disagree.
Many Embassies request a return/onward ticket to issue a TV.
Many airlines check for a return/onward ticket entering VE/TV.

17 hours ago, DrJack54 said:

For Tourist visa suggest that even if e-Visa application asks for exit date proof of actual onward ticket not required.

Also not required by airline at departure flight to Thailand

I have never been asked by the airline about return flights over the years when travelling with a Tourist visa.

When applying you do not have to supply proof of the flight ,only a flight number and proposed return date.

Visa exempt is a different story as have been regularly asked by the airline how many days will I be staying and if I have a return flight.

18 minutes ago, mlkik said:

Visa exempt is a different story as have been regularly asked by the airline how many days will I be staying and if I have a return flight.

Your whole post is also my understanding.

Some folk state that even flying visa exempt they are not asked for onward flight.

IMO seems the route and airline is big part of it.

When (for several years) I was returning from Saigon each month and I was asked every time by AirAsia.

No problem as I had a reentry permit.