Oxygen (O₂) is a key indicator of ocean health. The risk of O₂ loss in the productive coastal/continental slope regions is increasing. Autonomous underwater vehicles equipped with O₂ optodes provide lots of data but have problems resolving strong vertical O₂ changes. Here we show how to overcome this and calculate how much O₂ is supplied to the low-O₂ bottom waters via mixing. Bursts in mixing supply nearly all of the O₂ to bottom waters in autumn, stopping them reaching ecologically low levels.