Back in 2018, OneNote Online had a simple, dedicated toggle for hiding the notebook and section navigation panes so you could focus on a single page: a hamburger style icon in the navigation area that let you select Hide All. If you came here looking for that exact tip, it is worth knowing upfront that OneNote for the web has gone through significant redesigns since then, and that specific toggle is no longer guaranteed to be in the same place, or to exist in the same form at all.
The most reliable option today
Rather than chase an interface element that may have moved again by the time you read this, the most dependable way to get a distraction free, full width view of a OneNote page in the browser is your browser's own full screen mode, typically F11 on Windows. This removes the browser chrome entirely and gives OneNote Online the most screen space it can offer, regardless of what navigation controls the current version of the interface exposes.
For the current, exact steps
Because Microsoft updates the OneNote Online interface periodically, the authoritative source for the current navigation controls is Microsoft's own Navigate your notes with OneNote support page, which is kept up to date as the interface changes rather than describing a specific version frozen in time the way an older blog post like this one inevitably does.
For more current Microsoft 365 guidance, browse the rest of our blog, or if your team wants hands on help getting the most out of OneNote and the rest of Microsoft 365, our coaching and enablement service is built for exactly that kind of ongoing, practical support.