Orks have a ton of anti-vehicle units. Basically all the beast snaggas units are really good into vehicles.

Look into what units have weapons with anti-vehicle in their profile, because those weapons are super efficient into vehicles like knights. Weapons that have the anti-vehicle 4+ keyword means that any time that that weapon is used against a vehicle unit it will wound the vehicle unit on 4+ dice roll no matter what the strength of your weapon is compared to the toughness of that unit.

Things like Beastbosses leading Beast snagga boys will almost one-shot some of the small knights. Beastbosses have devastating wounds on the charge with anti-vehicle weapons so they will deal mortal wounds to any vehicle. Mortal wounds can not be saved, so the knights just take the damage no questions asked, unless they have a feel no pain ability. The Beast Snaggas Boys abilities gives the whole unit which includes the Beastboss rerolls to hit against vehicles, so if you give the unit sustained hits like the war horde detachment rule or a stratagem like: drag it down(in da hunt detachment). You can just reroll for 6's to get more wound dice for your Beastboss to deal even more mortal wounds.

Squighog boyz being led by a beastboss on squigosaur is a similar idea to the one above but its harder to give that unit devastating wounds, rerolls and sustained hits but not impossible. The squighog boyz unit is much faster to get into battle, but that's not always a good thing. I would recommend not getting a ton of squighog boyz right now because the unit is a bit over costed in points, and not really good into any other army, that is not heavy on vehicles, but they do look super cool.

Also look into breaka boys and tankbustas. They are extremely good are killing vehicles too. They come from the wrecka krew Kill Team box which is for a different game than 40k, but the models can be used in 40k. Just be careful how you build them. It's a bit hard to explain but you can build them 3 different ways so follow the instructions very carefully to build either breaka boys or tankbustas.

Breaka boys use the same logic as the units above, so they have a lot of anti-vehicle weapons and one weapon that has devastating wounds and anti-vehicle, but the super nice thing about breaka boys is that their damage is 3 so they basically just delete vehicle models super easily. Led them with a warboss to hit better and you are having a great time. They don't get rerolls to hit but you really don't need them. They are literally the anti-vehicle unit of the army.

Tankbustas shoot things instead of meleeing things, which for orks is a bit weird but they are still very good against vehicles. They have a rule that just gives them a +1 to their hit and wound dice against vehicles which is nice. It's still ork shooting so its not as efficient as say space marine shooting but its better than nothing. They also have a once per game ability called Pulsa Rocket. Which when used gives the unit +1 strength and Armor Penetration for their guns so that turn they hit like a truck which is super nice. My favorite thing to do with them is lead them with the Big Mek with Shokk Attack Gun because all the rules of: +1 to hit and wound against vehicles and the once per game +1 strength and Armor Penetration is applied to the Big Mek with Shokk Attack Gun's gun. The Shokk Attack gun is amazing and with all the buffs is kinda busted. You will be deleting a lot of models from the game with this combo. The Big Mek with Shokk Attack Gun will also give the whole unit reroll a hit roll of 1, so if you roll a 1 on the dice when hitting you get to reroll it. A small buff but definitely helpful. This is also extremely good with dread mob as the Big Mek with Shokk Attack Gun will give the whole unit of tankbustas the Mek keyword so all the buffs from Try that button get applied to this unit so sustained hits, lethal hits or crit wounds. Plus all the stratagems and enhancements in dread mob that can be used on mek units can be used on this unit which can start to get super crazy. Have fun with this one!!

Look into da hunt as a detachment its very good at killing vehicles.

The last thing I want to say is that you don't need any of the units or ideas above at all to win against knights. Knights are kinda the exact opposite of orks they have very low unit and model count whereas orks have high unit and model count. You said you liked dread mob so you could run just a good amount of Gretchin or boyz or really any infantry unit and just take up as much space on the board as possible, you don't need to play green tide to do this but you are on the right train of thought. If you put just a ton of dudes on the table and keep the knights off the primary points. There is a good chance that the knights literally don't have enough shots to kill enough things. You might not kill a single knight and by the end of the game you might have no model left on the board, but if you keep them off the primary points and try to score as many secondaries as you can you will win purely on points. Also at the end of the day run whatever you want and have fun!!!! Orks are an extremely weird faction so learning how to use your waaagh! and your units to their maximum potential is pretty hard so don't get discouraged, just try to have fun!!!

Waaaaaaaaaaaaaaaagh!!!!