You can generate a few types of reports in ArcGIS Online. This blog post will walk you through what's available.
You can also check the Credits tab in the Status section of ArcGIS Online.
You'll see a Credit Utilization Chart. This chart is interactive and allows you to drill down into different credit usage groups and some of them will allow you to download a report.
Example: I want to see a report of all the hosted feature storage.
1. On the Credit Utilization Chart I click on the Feature Storage slice
2. In the table below the chart, I click on the Standard Feature Data Store record
3. I then click on the View item details button
4. The Database Storage Details window pop-ups. I click on the download button to download a report showing all the hosted feature data in my org for the selected period along with their credit cost.
You can repeat these steps for various types of credit usage. Note that not all types of credit usage will allow you to download a report.
Credit usage is typically shown in 3 ways:
1. General credit usage associate to the org itself. It's not associated to a particular item or member.
2. Credit usage associated to hosted items like hosted feature layers and tiled imagery layers
3. Credits associated to members. Examples: Geocoding, Spatial analysis.