Tie the meat with kitchen twine if needed. In a small bowl combine the salt, pepper and oregano together. Sprinkle the seasoning mixture liberally and generously over the beef on all sides.
Heat the olive oil in a large skillet over medium-high heat. Add the beef and sear it on all sides until a nice brown crust has formed.
Add the remaining ingredients to your slow cooker and stir. Add the beef and pour some of the liquid over the meat.
Cook on HIGH for 2 to 3 hours or LOW for 4 to 6 hours. The meat will be done when the internal temperature reads 130-135 F degrees for medium-rare or 145 F degrees for medium. My beef was cooked to a medium, if you prefer more rare, check the meat after 2 hours on HIGH, or 4 hours on LOW.
Transfer the meat to a cutting board and cover with foil. Let it rest for about 10 to 15 minutes before slicing into it.
In the meantime, pour the remaining liquid from the slow cooker through a fine mesh strainer. Reserve both the liquid and the onions for sandwiches. Discard the bay leaf.
Slice the beef as thin as you can.
To assemble the sandwiches, slice the rolls in half then pile some beef on the rolls, then top with onions and a slice of Swiss cheese, if preferred. Place the rolls under the broiler, if preferred, for a couple minutes until the cheese melts.
Serve them immediately with the reserved liquid/au jus to dip the sandwiches into.