The short answer
A fake Omega caliber 9900 is a costume, not a copy. The movement inside is usually a Valjoux 7750-type clone or a generic Chinese automatic chronograph, dressed with an engraved rotor and a hopeful laser-etched bridge plate. It carries the words Co-Axial Master Chronometer because those words cost almost nothing to print. The real 9900 carries them because it passed METAS certification, survives 15,000 gauss, and runs a column wheel with a vertical clutch. Two very different machines sharing one case back.
If you have one in hand and want a verdict in five minutes, do three things: time it, loupe it, and put a magnet near it.
What the genuine 9900 actually is
Omega's 9900 belongs to the 9000-series family of chronograph movements. Automatic winding, twin barrels in series for a 60-hour power reserve, a Co-Axial escapement, a silicon balance spring, and magnetic resistance rated at 15,000 gauss. The chronograph side runs a column wheel and a vertical clutch, which is why the pushers feel crisp and the chrono seconds hand starts without a visible jump.
On the dial side, that layout shows up as central chronograph minutes and hours, running seconds at 9 o'clock, and a date. If a watch claims a 9900 but stacks its counters some other way, the case is lying about the engine.
Finishing is where the real money goes. Arabesque Geneva stripes on the rotor and bridges, blackened screws, polished bevels that catch light in a thin line, engraving you can feel with a fingernail. The Si14 mark on the balance bridge. A free-sprung balance with no regulator index poking out of the cock.
What ends up inside a "9900" replica
Most of these watches hide a 7750 clone or a rebuilt ETA-style chronograph behind the display back. Some use a stamped plate bolted over the top, so the escapement you see is a decorative ring rather than a working part of the watch. Beat rate gives it away first: the movements the copies borrow from usually tick at 28,800 vph, while the 9900 runs at 25,200.
A few sellers advertise a loose "Omega 9900 movement" for sale. Omega has never sold this caliber into the parts market. A loose one does not exist, which makes that listing a fake by default.
Checks that separate them
- Beat rate. A timegrapher should read 25,200 vph. 28,800 or 21,600 means it is not a 9900.
- Magnet test. A real one keeps time near a strong neodymium magnet. A steel hairspring copy will gain or lose minutes, or stop dead.
- Regulator index. There is no regulator arm on the balance cock of a genuine 9900. Nearly every copy has one.
- Chronograph start. Column wheel plus vertical clutch gives a clean start and a snap-back reset. A spongy push and a lazy reset point to a cam and a horizontal clutch.
- Engraving depth. Real text is cut, not printed. Run a fingernail across "OMEGA" on the rotor. Flat means laser.
- Stripe pattern. Arabesque waves curve in a specific sweep. Copies use straight, even striping that looks like it came off a printer.
- Screw and jewel color. Real screws are blackened and polished. Copies often show bright chrome or painted purple jewels.
Why it matters to a buyer
If you want the look, a decorated clone tells time and looks busy through the back glass. If you want the movement, there is no shortcut. Service is the other end of the problem. A real 9900 can be repaired by Omega. A clone with a fake plate over it usually cannot be serviced by anyone, because the parts under the plate were never built for that layout. Chrono functions on the copies fail first, and the reset hand often stops returning to zero within a year of regular use.
Buy the watch, not the badge on the rotor. The badge costs nothing to fake. The 60 hours, the magnet spec, and the column wheel are what you are actually paying for, and none of them come in the replica.