Here are some YouTube channels having research level talks, all of which were livestreamed with live Q&A prior to being archived. These are all ongoing series with new talks (videos) added regularly.
Discrete Optimization Talks
One World Optimization Seminar
Communications in NLA (Numerical Linear Algebra), which has many talks pertinent to optimization, as well as some in optimization per se.
Given the research level of these talks, they are more generally better suited for people who have already found their way to Operations Research, or at least a closely neighboring field, than for neophytes.
Euro Working Group on OR Practice
Edit: Additional seminar series:
Machine Learning NeEDS (Network of European Data Scientists) Mathematical Optimization This series includes a number of presentations from leading academics in the field of Data Science and Analytics that will cover important topics such as explainability, fairness, fraud, privacy, etc. Mathematical Modeling and Mathematical Optimization will be at the core of their presentations. We also have the YOUNG Online Seminar Series “Machine Learning NeEDS Mathematical Optimization”.