Identifying the oxygen atoms that undergo reduction
The oxygens in the dichromate ion on the right have -2 oxidation number, the chromium being +6.
http://ch302.cm.utexas.edu/worksheets/basic-redox-wkst-KEY.pdf
https://socratic.org/questions/what-is-the-oxidation-state-of-chromium-in-the-dichromate-ion-cr2o7-2

As you seem to have deduced, the peroxide ions are -1 so all oxygens, as shown, have been reduced.