The oxygen mask will be used in conjunction with the magazine found in the first suitcase.
Two pages are dog-eared in the magazine. Do the symbols look familiar?
There is only one symbol on the mask that matches the symbol at the top of the circle on the right page. Align those symbols.
On the top of the left page, there are three symbols.
Once the mask is aligned, find the matching three symbols from the left page on the mask. Use the corresponding numbers to find the code.