Severus kept his amusement at Oliver's reaction carefully concealed. He waited until Oliver turned around then cleaned the liquid that had been spilled with a flick of his fingers. "Do pay attention, Oliver. Never assume you are safe, especially when among Slytherins. Many of whom could charitably be described as 'murky'." He included himself in that description.
He arched an eyebrow at Oliver's outburst and the mumbled comment and his expression cooled slightly. "I am aware of your successes with the vaccine and treatment and I trust you have written your procedures up clearly and concisely. However if you wish to attain your mastery, resting on your laurels is not the way to do it. Nor should you require constant supervision from your master. You are not a schoolboy. Part of achieving one's mastery is demonstrating the ability to work independently as well as in collaboration with your master."