Both O33 and O36 are official stamps of 1920-21 (official stamps are stamps issued for use only on government mail) overprinted 800 thousand on 30 pfennig in 1923 during the hyper-inflationary period in Germany.
The difference is in the watermark.
Scott O33 has watermark 125, which is a lattice of diamonds:

Scott O36 has watermark 126, which is a fishnet pattern:
