PTO stands for Patent and Trademarks Organization.
PTSO is not a recognized abbreviation in terms of patents as far as I know.
PTSO stands for Patent and Trademark Support Organization. It refers to a professional organization or group that provides support, resources, and networking opportunities for patent secretaries and professionals in the intellectual property field.
According to "Understanding Chemical Patents" the Patent Secretary is the Company Liaison between the researcher/inventor and the PTSO. An inventor in the company employ would go to the patent secretary in order to see if the invention was patentable and if so the patent secretary in some cases directly files on behalf of the inventor/company or submits it to the Board or committee for modification/review.