KAMI : The highest ranking grade in Aikikai is 9th Dan (the one and only 10th Dan was Koichi Tohei Sensei when he was still a member and Head Instructor of Aikikai Hombu Dojo).... [/b]
According to AikidoFAQ
Osawa sensei was given 10th dan after his death in 26.05.91, given post mortem by the Aikikai. Rinjiro Shirata sensei was also given a post mortem 10th Dan. This surprised me. You tend to forget about post mortem promotions.
Minoru Mochizuki 10th dan - given by the International Martial Arts Federation (IMAF) in 1979. There were also several other people who received 10th Dan by IMAF.
I think you can safely say
Koichi Tohei Sensei was the only one to receive a 10th Dan certificate from O Sensei.