Can I post on Arxiv a final paper published on PLOS ONE?
Yes, you can.

Read the CCBY license: you are allowed to share (copy and redistribute the material in any medium or format) and adapt (remix, transform, and build upon the material
for any purpose, even commercially) the material as long as you attribute (you must give appropriate credit, provide a link to the license, and indicate if changes were made). So you can put the paper on arXiv, as long as you attribute.