All of this added up to 41 (T1) to 43 (T2) tons, at the extreme lower end of MBT weights of the period; this is why a smoothbore gun was preferred over rifled, as it’d generate less recoil on an already lightweight vehicle. It was 10,1 m long, 3,26 m wide and 2,37 m high -like the Tamoyo, an advantageous low silhouette.
There were several proposed variants, such as a 155 mm howitzer, AA vehicle with two 35 mm guns and an armored recovery vehicle. The howitzer would require cooperation with Voest-Alpine.

The first chassis was finished in September 1984. It received a fake turret and was tested for movement, resistance and so on with some changes being made. Vickers delivered a 105 mm turret in May 1985, which was immediately equipped, and the prototype was flown to Saudi Arabia in JUly for pre-tests. There it met its formidable competitors: the M1A1 Abrams, Challenger 1 and AMX-40.
The Challenger suffered an engine meltdown during tests. This inspired a new system in the Osório: if its water and oil temperatures got too high it automatically lowered engine power.
Performance was overall reasonable, though Saudis complained about the 105 mm gun and engine deficiencies were shown, and later solved at home.

The 120 mm turret arrived in early 1986. For the Brazilian army, tests with the 105 mm turret were conducted in December – April with 3269 km of movement, among them 750 in irregular terrain, and the gun was fired 50 times. It fulfilled all of the military’s requirements.
Next came the crowning moment of Engesa’s existence, which would give the Osório an everlasting reputation and has since been the object of hearsay and controversy. The four competing tanks were once again taken to Saudi Arabia for the final batch of tests from July 8th to September 12th 1987. As Engesa’s engineers were working until the last moment there wasn’t enough time to ship by sea and the Osório was flown by Air France.
Each participant was given one month to train a team of randomly selected Saudi personnel which then had to operate their platform in:
-2350 km of movement: 200 km for training, 400 km in highways and 1750 km in the desert, with a maximum fuel consumption of 2,1 km/l on the desert and 3,4 km/l on roads.
-Overcoming 3 m wide trenches, starting up movement on a ramp with 65% inclination and other tests on an inclined plane.
-40 minute tread removal and replacement, 6 hours of engine operation with the vehicle standing still, 6 km of backwards movement and towing a 35 ton vehicle for 15 km.
-82 shots with both vehicle and target stationary and a 4000 m distance and 67 with a 1500 m distance and target/both vehicle and target in movement.
Testing grounds included Sharouah and Khamis Mushait, the former in high temperature conditions.
The Challenger suffered an engine meltdown again.
On the final gunnery test the Osório achieved the highest hit rate, followed by the Abrams, Challenger and AMX-40. It did, however, suffer some damage in the rubber of some ball bearings in the treads. It also managed to tow the Abrams which was much heavier than itself.