The Osa is a high speed missile boat of Soviet origin. It was developed in the early 1960's as a more capable alternative to the Komar class missile boat. The Osa is the world's most numerous missile boat. The official Soviet designation for this class is Project 205 Tsunami. The name Osa, which Russian for wasp, is the NATO designation. The improved Project 205U is known as Osa II.

The Osa class is a major improvement over the Komar missile boats. The Osa class is four times larger, is based on a steel hull, features 4 instead of 2 missiles, has a much improved air and self defense capability and better sea going performance. The improved Osa II features better engines and can fire a newer version of the P-15 anti-ship missiles. The main armament of the Osa are the four P-15 (SS-N-2 Styx) anti-ship missiles. Air defense is provided by two AK-230 turrets with two 30mm autocannon. One turret is fitted forward and the other aft. The AK-230 turrets were unmanned, each armed with two 30 mm guns capable of firing 2000 rpm (400 practical) with a 2500 m practical range. For an increased air defense capability the manually aimed quad launcher for the infra red guided Strela (SA-N-5) surface to air missiles was added to some vessels, mostly the Osa II.

To keep the design simple the Osa has three diesel engines each connected to its own shaft. The power to weight ratio is high and results in a maximum speed of 39 knots. The Osa II has more powerful engines and a slightly high maximum speed. Although the sea going capability is better than the Komar the Osa is not a true ocean going ship and is mostly used in coastal waters.

The Osa was widely used by Soviet and Eastern European forces during the Cold War. It was also widely exported, mainly to Asia and the Middle East (over 400 were made in USSR, and another 120 in China). These missile boats saw action during the War of Attrition, Yom Kippur War, and Indo-Pakistani War of 1971, also used in the Iran-Iraq War.

Missile boat R-536 was built at the Rybinsk Shipyard (yard No.686).

Was listed as missile boat of the 41st Brigade of Missile Boats. 29 January 1971 the missile boat was transferred to the Indian Navy. There he got the name INS Nirghat (K89).

During the Indo-Pakistani War of 1971, INS Nirghat was part of the Operation Trident strike force, along with her sister vessels from the 25th Killer Squadron (INS Veer (K82) and INS Nipat (K86)). On the afternoon of 4 December 1971, the strike group made its way towards Karachi. Late that evening, around 70 miles south of Karachi, Nirghat detected a large Pakistan Navy target, later identified as the destroyer PNS Khaibar, about 45 miles to her northwest. Nirghat launched two P-15 missiles at Khaibar, sinking her.

In 31 July 1989, the missile boat was excluded from the lists of the Indian fleet.