Here's a quick and dirty guide to DDR officials:
SC O1 -17 have dots in the background:
The remainder of officials have a lined background:
Now the difference between between the O18-O27 series and O37-O43 is paper and watermark.
O18-O27 is on plain paper with a 297 watermark. The O37-O43 series is on granite paper with a 313 watermark. The easiest way to distinguish between the two series is the paper. Granite paper looks like this:
Now for the trip-ups:
The O22 is on plain paper with the arc pointing left:
The O22a has the arc pointing right:
The O22a is identical
to O40 except for the paper and watermark. The O40 is on granite paper with a 313 wmk and the O22a is on plain paper with a 297 wmk. You have to be careful checking for granite paper and watermarks on O22a's being offered.
Now, if you want to get further into the weeds, O37-43 have two different perforations: 13 X 12 1/2 and perf. 14. There are also O18, O19, O21 and O25 issues that were printed on granite paper. The only way to distinguish them is by watermark. They have a 297 watermark whereas the other granite paper printings have a 313 wmk.
To get even deeper into the weeds, there are also plate flaws that are quite collectable. Some collectors also collect by the direction of the watermark.
Hope this helps!