"On our 6!" 

The OSV's were not terribly fast, built by the Imperium for reliability rather than speed or armor. Top speed on a flat surface with enough time, they could reach up to about 125 KPH, and that was without passengers or ordinance. The body is constructed from lightweight and rust-resistant aluminum, instead of conventional steel, at least in it's most common variant. Small caliber rounds from autoguns and the occasional lasbolt or three wouldn't take the vehicle out of action. You'd really need to hit it in the right spot with a bolt pistol or high powered rifle to damage it. However, they were pretty much just moving targets for Orks with their heavy armor and high caliber guns.

"Jumpers!" 

"Where did they come from!?" 

Zeb should have expected this, his lasgun raising with his head as the drivers nearly lost formation from the surprise flanking. Behind them, heavily armored wartrukks had erupted out of a gulley, ork boys screaming to their twin gods and guns firing sporadically. Men cursed or cried out to the Emperor, but to their credit they recovered well, Zeb ordering them to fire even as ork stormboyz clinging to the grills of the wartrukks screamed and leaped, their jump packs roaring to life just as gravity was about to take them, flinging them towards the convoy of OSV's. A few orks flew into the distance, unable to take proper control of their equipment, but a few managed to land in the midst of the trucks. A stormboy landed in front of the left wing of the convoy and was hit full on by one of the vehicles, cracking the windshield and breaking the bumper, knocking the ork flat. A testament to its species, it was still alive until the third truck ran it over. 

Zeb had the misfortune of his stormboy landing atop the hood of his OSV, the xenos' choppa nearly decapitating him with a vicious swing. Zeb blocked with his lasgun, the choppa biting into the butt of the weapon and knocking it out of his hands, flinging Zeb back into his seat. He kept his wits about him and reached for the backseat, taking out an autopistol. Normally it wouldn't be very effective against a hulking Xenos like an Ork, but seven rounds fired in quick succession to the face was enough to take it out, breaking its goggles in spurts of blood and sending it falling off the truck. Zeb just managed to grab his lasgun from the choppa before it fell onto the road. 

Relatively small rounds (for orks) erupted from the forward machine guns from the three ork wartrukks behind them, biting into the back armor of the OSV's. Heavy stubbers burst into life, roaring like carnodons. One man to Zeb's left lost his head, the shrapnel from his skull killing the man beside him, the bone fragments slicing into his neck. Another OSV was hit by a stormboy, the ork landing in the midst of the men and cracking open the driver's skull with its cleaver. The truck veered, hitting another OSV and nearly running it off the road, but it kept its course and instead the ork one suddenly spun to the right, flying into a ditch that threw the ork and the men out at terrible speed. 

Zeb's lasgun cracked, firing at full auto at the orks in the wartrukks, one of the vehicles nearly punctured like swiss cheese and another with a blown wheel. Shotguns roared, slugs slamming into the orks, taking off chunks of their meaty shoulders. The next minute was naught but a furious exchange of shells and lasers, until there were only eight OSVs left, and the last wartrukk was dismantled, its tires blown out and the orks pitching childish fits. 

Zeb wiped the sweat from his brow, a dozen men or more under his command were now dead. He gave a quick prayer to the Emperor to guide their souls, about to shoulder his lasgun when the call came over the vox that a shuttle had been spotted ahead, and lasguns were being fired from within. The only shuttle that could be out here as far as he knew was Katia's, but even if it wasn't, they had a duty to help. 

"A hundred greenskins are assaulting their fortified position, sergeant." 

"Hit them hard from behind, now!" Zeb ordered, and the heavy stubbers were hastily reloaded as the OSVs moved into position to hit the Orks in a lightning strike they wouldn't soon forget.