MOS

MOS stands for Modular Optic System. It refers to a factory modification of the Glock frame and slide, allowing easy installation of a red dot sight directly on the slide without the need to alter the construction. MOS is Glock’s proprietary optic mounting system, while OR is a general term used by other manufacturers, including CZUB. Both systems serve a similar purpose but differ in adapter variety, compatibility, and labeling. Airsoft replicas often mimic the MOS or OR design with a pre-cut slide for red dots (e.g., CZ P-10C GBB by ASG or Glock 17 Gen5 MOS by Umarex).

The MOS system typically includes:

A specially milled slide allowing for interchangeable mounting plates.
Multiple adapter plates for various red dot footprints (e.g., RMR, Docter, Shield RMSc, etc.).
The slide is often configured so that iron sights remain functional when using a red dot (co-witness).

CZUB and the OR label

CZUB (Česká zbrojovka) uses the designation OR, which stands for Optics Ready.
For example, models like the CZ P-10 C OR or CZ P-09 OR feature a milled slide for direct red dot mounting.
The main difference from MOS is usually in the adapter system used or the number of adapters included – the MOS system tends to be more universal (more plates in the box).

Main difference between MOS and OR

Feature	MOS (Glock)	OR (e.g. CZUB)
Slide	Milled for adapter plate	Milled for adapter plate
Adapters	Included in box, multiple types (e.g. RMR, RMSc, Docter)	Sometimes only one adapter or optional accessory
Compatibility	Higher – supports more red dot brands	Limited – often for specific red dot types
Standardization	Unified Glock system	Each manufacturer has its own solution