Totally legit Omega in a very nice silver Art Nouveau case. Dial with Empire numerals and hands match the period. Serial number dates the watch to about 1920 or so. Possibly a caliber 17''' L ? @OMTOM @François Pépin may shed more light?
Niello decoration on the case back. The case is .900 (coin) silver. A quality watch. Surprisingly only 15-jewel movement. Enamel dial looks in nice shape.
Legit, yes, and not so common! Nice find! The movement seems to have a nice silver finish, maybe to match the case, which is unusual for those 19' series like calibres - they usually have a gold finish unlike the later calibers such as the 35,5. What is the size of the movement?