Sending sensitive information via email between internal users in Office 365 - is it safe?

Yes, it is, this is called encryption at rest and in transit, this means without having to do anything data in Office 365 is protected.

See plenty of details here - https://support.office.com/en-ie/article/encryption-in-office-365-0a322724-08ca-43db-b69a-afbfa20484cd. However, for different reasons you still might want to employ additional measures to protect confidential data, especially when it's being sent externally.  Here are some ways of doing that:

https://support.office.com/en-us/article/office-365-message-encryption-ome-f87cb016-7876-4317-ae3c-9169b311ff8a

"With Office 365 Message Encryption, your organization can send and receive encrypted email messages between people inside and outside your organization. Office 365 Message Encryption works with Outlook.com, Yahoo!, Gmail, and other email services. Email message encryption helps ensure that only intended recipients can view message content."

https://support.office.com/en-us/article/introduction-to-irm-for-email-messages-bb643d33-4a3f-4ac7-9770-fd50d95f58dc

"Information Rights Management (IRM) allows you to specify access permissions to email messages. IRM helps prevent sensitive information from being read, printed, forwarded, or copied by unauthorized people. IRM also helps organizations enforce corporate policy governing the control and dissemination of confidential or proprietary information, both within the organization and with customers and partners."