To review Schedule O Supplemental Information to Form 990 or 990-EZ:
Per IRS Instructions for Form 990 Return of Organization Exempt From Income Tax:
On page 4:
C. Sequencing List To Complete the Form and Schedules
In general, first complete the core form, and then complete alphabetically Schedules A–N and Schedule R, except as provided below.
On page 83:
Appendix E. Group Returns—Reporting Information on Behalf of the Group
In general, if a line requires a Yes/No answer and the answer isn't the same for all subordinate organizations to which the line applies, then check “Yes,” and explain the answer in the schedule's supplemental information section (if applicable) or in Schedule O (Form 990 or 990-EZ).
Note that any link in the information above is updated each year automatically and will take you to the most recent version of the document at the time it is accessed.