Do I need to cancel my current visa (visa O) before applying for a DTV?

A non-O visa is only 90 days. Just let it expire. No need to cancel

When transitioning from a Non-Immigrant O visa to DTV, you effectively need to ensure that you're not holding two active visas simultaneously, which means you might need to cancel or let your current visa expire before applying for the DTV. Some experiences suggest that entering Thailand on the new visa could automatically cancel or deactivate the previous one, especially if no re-entry permit is attached to the old visa. However, for clarity and to avoid any administrative hiccups, it's recommended to either formally cancel your existing visa or confirm with Thai immigration or an embassy that your visa status is clear before proceeding with the DTV application. Remember, consulting with a legal expert is advisable for personalized advice. 🙏

If you don't have a reentry permit it will be cancelled the moment you stamp out of the country.