What is the difference between Ostracodermi and Placodermi
If in reference to the prehistoric groups of fish of the respective classifications, ostracoderms would have had a bony head much like a shell, and placoderms had articulated plates covering the head/thoracic area. The differences are relatively straightforward, see the wikipedia pages for each:

https://en.wikipedia.org/wiki/Ostracoderm

https://en.wikipedia.org/wiki/Placodermi