Many in very important subsystems. The XFS maintainer (who just recently stepped down from this role) and contributor Darrick Wong works for Oracle. Meanwhile, btrfs is the creation of Chris Mason, who worked there until 2012. Modern filesystems on linux owe a decent debt to Oracle. Good luck running an Oracle-free linux.
I often find it interesting when people imply the company is freeloading for having chosen the path of making an RHEL clone. They cloned RHEL because an unhealthy, dependent ecosystem was built around the one, singular linux distribution, not because they are unwilling to fund work.
Giving some serious competition to Red Hat can only be a good thing.