Office 365 (Web) – Disable Conversation View in Outlook
Outlook on the Web defaults to “Conversation” view which groups all related emails sharing the same subject line together. While some users find this feature convenient, others find it difficult to locate and respond to certain emails quickly as it collapses the content within a conversation thread.
Select the Settings icon in the upper-right. In the popup that appears, in the Conversation view section, select Off.