In 1952, Sultan Omar Ali Saifuddien who had taken over the throne on the death of his elder brother, issued the first definitive stamp with his portrait. The previous Sultan, Sultan Ahmad Tajuddin did not issue any definitive stamp but did issue a special commemoration in 1949 to celebrate his 25th Silver Jubilee on the throne.
The Sultan Omar Ali series were issued from 1952 to 1971. Three definite series can be determined. The first series was issued in March 1952 on papers with watermark multiple script CA and a perforation of 13.
The second series was issued from 1964 to 1968 with the only difference with the earlier one of the watermark changed to Block CA. The other difference is that the second series only issued stamps up to $1 value.
Despite Sultan Omar Ali abdication of the throne in 1967, the third series of definitive stamp using his portrait continue with the third series issued from 1969 to 1971. This time the paper was changed to glazed paper but the watermark was unchanged still using Block CA and the perforation remaining 13.