Dear Dr. Cruz,

You can find these information on the dictionary available from OASIS (https://bpb-us-w2.wpmucdn.com/sites.wust...).

Days from entry is one of their way to identify their dataset on the file name, i.e.: OAS30001_FDG_d3746, where 30001 is the patient ID, FDG is the modality, and d3746 is the day when the data was acquired since the subject entered the study. So if you have OAS30001_rsMR_d3747, it means the same subject, rsMR as the modality, and d3747 is 1 day after the FDG was acquired, on the 3747th day after the subject entered the study. 

I hope this answer helps you. 

Best regards,

Phelipi Schuck