Here's how to tell your staff to start using OASIS C. Question: How do we know when to use OASIS B-1 and when to use OASIS C? Answer: Home health agency staff may mistakenly think they need to use the version of OASIS in effect when a patient started care. Look to M0090 (Date assessment completed) to determine the correct start date, said Deb Deitz with the Centers for Medicare & Medicaid Services contractor Abt Associates. If you're doing an assessment and the M0090 date is Jan. 1, or later, then you need to be collecting OASIS C data, no matter if it's a start of care, resumption of care, follow-up, transfer, or discharge, a new patient on service or a patient continuing on service, Deitz said in CMS's "Train the Trainer" conference call on Oct. 22, 2009.