Short answer: Omega does not list a caliber 9800 anywhere in its documentation. The modern Omega families people actually talk about are the 8800 and 8900 series for three-handers, the 9300 and 9900 for chronographs, and the 8500 generation before those. When a replica listing advertises a "Caliber 9800," the number is a label built to sound in-house, and the thing ticking inside is nearly always a mass-produced automatic from a Chinese or Japanese factory, decorated to look the part. Judge the watch by the movement you can inspect, not by the name on the rotor.
Why the 9800 number shows up at all
Replica catalogs borrow Omega's numbering style because buyers recognize it. Four digits, a "Co-Axial" line on the rotor, a few Geneva stripes, and a $180 movement starts reading like a $6,000 one in a listing photo. The tell is paperwork. Genuine Omega calibers are documented: jewel count, power reserve, magnetic resistance, service specs. You can find all of it. A 9800 has no spec sheet, no certification record, no service history from the brand.
What is usually under the case back
Most often one of three things: a Seagull or Hangzhou automatic in the 2824 family, a Miyota 9015, or a decorated Chinese chronograph module. All three can keep decent time. None of them are co-axial. The co-axial escapement is a specific geometry with three pallet jewels instead of two, and no factory is reproducing that for a watch that sells in the hundreds. If a seller claims co-axial and the balance is a standard Swiss lever layout, the claim is cosmetic.
How I check a movement before buying
- Ask for a clear photo of the balance wheel. A free-sprung balance has no regulator arm with two pins. Many decorated clones keep the regulator, which gives it away.
- Check the beat rate. The 8900 family runs at 25,200 vibrations per hour. Plenty of clones run 28,800. Not proof on its own, but a data point.
- Look at the engraving. Laser etching on a clone rotor reads flat and slightly gray. Real Omega finishing has depth and a crisp edge.
- Ask for a 10x video through the case back, not a marketing render.
- Ask whether a Master Chronometer card exists. Certified watches ship with one, and the number can be checked.
Price, service, and water resistance
A decorated automatic in a replica case will go a few years between services, and any watchmaker who works on 2824 clones can handle it. Budget for that. Water resistance is where replica claims fall apart fastest. A "300m" rating on an unsealed case with a push-in crown is wishful thinking, and I would not swim with one. If you need a diver you can take in the ocean, buy from a brand that stands behind the rating.
The honest caveats
A replica is not a genuine Omega and cannot legally be sold as one. Customs in the US and the EU seize counterfeits on a regular basis, and the Swiss watch industry tracks the trade closely. If you buy one, buy it as what it is: a look-alike with a generic movement, no brand service, and no resale value. Anyone telling you the 9800 is an Omega in-house caliber is repeating a seller's copy or guessing.