Oma water is directly related to the Banda Sea, which causes this water to have the highest diversity of fisheries resources. Oma waters is a potential fishing ground for small and big pelagic fishes. One of the most important pieces of information on the abundance of small fishes in this area is the abundance of phytoplankton. Until now, there was not any information available on phytoplankton species composition as well as its abundance. The objective of this research was to study the species composition of phytoplankton, its abundance and, some ecological indices. Sampling was carried out in April 2017 at a 14-fixed sampling station using Kitahara plankton net. Sampling was conducted vertically from 20 meters depth to the surface water. Hydrological parameters such as temperature, salinity, pH, and water transparency were measured in-situ. Nitrate and phosphate were analyzed in a chemical laboratory in LIPI-Ambon. The total of 65 phytoplankton species was identified from the study area and can be grouped into Bacillariophycea, Dinophyceae, and Cyanophyceae. Bacillariophycea has dominated this phytoplankton species composition (70.70%). A total abundance of phytoplankton in this study area varied from 1.09 × 105 cells l -1 to 5.70×105 cells l -1. The five dominant species found in this study were Guinardia flacida, Thalassionema nitzschioides. Skeletonema costatum, Thalassiosira gravida, Chaetoceros curvisetus, and The Shannon diversity index (H’) varied from 1.447 to 2.774. The evenness index (J’) was between 0.404 and 0.738. The dominance index was from 0.262 to 0.596. Phytoplankton abundance had a positive correlation with water transparency and salinity, whereas the number of phytoplankton had a positive correlation with seawater temperature and nitrate concentration