The detection, analysis and modeling of seismic processes worldwide very often requires the use of ocean bottom seismometer (OBS). Indeed, the largest earthquakes and transient slow slip events (SSEs) mostly occur offshore or near oceanic coasts, along the major plate boundaries, and in particular on the interplate thrust fault of subduction zones. However, most OBS deployments are done with stand-alone stations, with data recovery delayed by months or years. On the other hand, electrically cabled OBS, which allow for real-time monitoring, like in Japan (DONET), USA(Neptune), or France (EMSO Ligure), remain exceptional due to their very high cost of manufacturing, installation, and maintenance. Here we present a new perspective for such cabled array of OBSs, using innovative, purely optical seismometers, plugged at the end of long fiber optic cables, aimed at reducing the cost of installation and maintenance for permanent observatories requesting real-time data. The optical seismometer has been developped in the last decade by ESEO, based on Fabry-Perot (FP) interferometer, tracking at high resolution (rms 30 pm) the displacement of the mobile mass of a 10 Hz, 3 component, purely mechanical geophone (no electronics nor feedback). A prototype has been successfully installed at the top of La Soufrière volcano of Guadeloupe, in 2019, at the termination of a 1.5 km long fiber. We replicated and marinized this sensor, and in June 2021 we installed it 5 km offshore the Les Saintes islands (Guadeloupe, Lesser Antilles), for better characterizing the intriguing swarm-type activity still persistent after the 2004, M6.3, Les Saintes destructive earthquake (Interreg Caraïbe PREST project). The cruise, FIBROSAINTES, was supported by the Flotte Oceanographique Française (FOF) with the N/O Antéa Research Vessel. A plow, designed by GEOAZUR, carried the reel with its cable, and was pulled on the sea floor by the vessel, burying the cable. The seismometer was installed by divers at the depth of 43 m. The landing cable in the harbour of Terre-de-Bas was connected to the interrogator, and the record are since telemetered in real-time to the Observatory of Guadeloupe (OVSG). The optical OBS has been qualified by comparison to the records of the M7.5 Haiti 2021 earthquake from a nearby land-based broad-band seismometer. A few local earthquake swarms have been recorded, allowing for a preliminary discussion of their mechanical origin. In 2024 the seismometer should be complemented by two other ocean bottom, innovative, high resolution instruments, a pressiometer and an hydrostatic tiltmeter, designed by ENS. This successfull installation opens promising perspectives for the seismic real-time monitoring in many other sites offshore, and more generally in any site, natural or industrial, presenting harsh environmental conditions, where commercial, electrical sensors are difficult and/or costly to install and to maintain, or simply cannot be operated. Cite this article as (2022). First installation of an optical OBS, cabled offshore Les Saintes, Lesser Antilles, Seismol. Res. Lett. XX, 2-24, doi: .