Coach Jack Del Rio provided the video evidence:
Yes, it's only players in shorts during non-contact drills, but it's fun to watch Beast Mode blast through a gaping hole and turn a DB around a few times before his fake leap into the end zone. Football is entertainment, and that's entertainment, whether or not it's a signal of anything substantial in the future.
That Del Rio is willing to share his practice video with the masses tells you all you need to know about the hype surrounding Lynch joining Oakland.
As for Beast Mode, he looks spry enough in that video, with good burst on the second level. With Lynch sitting out several previous OTAs, his participation is certainly noteworthy.