Board Thread:Theories and Speculation/@comment-1872728-20200725151448/@comment-32685487-20200730040208

Oro created the Cursed Seals after he himself acquired Sage Mode and by extension Senjutsu through training with the White Snake Sage, which since Oro was not eaten means he survived and since Oro can use Senjutsu means he is a Sage(again his host bodies cannot handle it making him an imperfect Sage). He then used his new Sage abilities and Senjutsu and combined it with Jugo's enzymes and made the Cursed Seals. He then applied the one to Anko Mitarashi, and then Kimimaro, and then to the Sound 4. Anko was the first succesful Cursed Seal user hence the rule of 1 in 10 and compatibility(I believe it was 1 in 10) surviving the process. Oro had Kimimaro and Jugo since before Part 1 began, since they were little kids, he put the Cursed Seal on Anko before he left Konoha.