Oyster Perpetual
Rolex's foundational watch family name combining the Oyster waterproof case (1926) with the Perpetual self-winding rotor movement (1931). Used as both a model name and an architecture designator across the modern Rolex catalogue.
"Oyster Perpetual" is two patented Rolex inventions joined into a single phrase that names the foundation of the modern Rolex range.
Oyster: the waterproof case
The Oyster case was patented in 1926 and was the first commercially successful waterproof wristwatch case. Its hermetic screw-down crown and screw-down case-back made it tight enough that Mercedes Gleitze swam the English Channel wearing one in 1927 with the watch working at the end of the crossing.
The Oyster case design persists today, with refinements (modern Oyster cases are tested to depths from 100 m on dress models to 3,900 m on the Deepsea Challenge), but the screw-down crown and case-back architecture is unchanged in principle.
Perpetual: self-winding
The Perpetual rotor was patented in 1931 and was the first reliably efficient self-winding mechanism using a 360-degree pivoting weight. Earlier automatic watches existed but used smaller bumper rotors with limited winding efficiency.
Every modern mechanical Rolex movement uses a perpetual rotor descended from the original 1931 design.
As a model name
"Oyster Perpetual" appears on most Rolex dials underneath the brand name, indicating that the watch uses both inventions. It is also a specific entry-level reference family (Oyster Perpetual 36 reference 126000, Oyster Perpetual 41 reference 124300, and similar) which represents Rolex's simplest no-date, no-complication line.
Common confusion
A Submariner is an Oyster Perpetual. A Datejust is an Oyster Perpetual. A GMT-Master is an Oyster Perpetual. The phrase describes the architecture, not exclusivity to the entry-level OP reference family. When a Rolex listing says "Oyster Perpetual" without further qualification, it usually means the simple OP reference family (no date, no rotating bezel, no chronograph). When it appears with another designator (Submariner, GMT-Master, Datejust), it identifies the architecture inherited from those two patents.