Hello,
Decision made status is updated once you pass your Citizenship test/Interview and the Citizenship officer has made their decision to grant or revoke your Canadian citizenship. The officers are supposed to tell you during your interview about their decision in regards to your application and once that's done they update it on the ECAS system.
And as for the Oath ceremony, you will receive a notice 2-3 months after you pass the test/interview. Rest assured your oath notice will be within the 12-month Citizenship grant processing timeline. For example: If you applied in May 2018 your Oath Ceremony will be held in May 2019 or earlier.
Hope this helps.