(or as we like to call it, the only time oxygen is allowed to touch wort!)
We all know oxygen is needed for healthy fermentation (except in the case of dry yeasts, i.e. they come with enough reserves that they do not need it). I would target 8-12 ppm of oxygen in the wort after yeast has been pitched. Here is a video we did showing you what it really takes to get wort to the proper ranges using pure oxygen, and verifying with a DO meter.
In the video I use a flow rate of 0.25 l/min. It takes nearly 4 minutes of pure oxygen and stirring to get to 11.5 ppm of dissolved oxygen! Stirring or splashing has only netted us 2-3 ppm DO, in our trials, so its not nearly enough! While we will never explicitly say a DO meter is needed, it will certainly help you in all aspects of Low oxygen brewing!
Here is what the pros have to say about oxygenation: Oxygenating Wort – Tips from the Pros
Good luck and happy fermentations!