Off the ground (OTG) combos are one of several types of combos in Super Smash Bros. Clash. These are very specific types of combos that are done on character lying on the ground. However, unlike normal combos, they are escape able by rolling in a direction, but NOT by simply getting up or doing a wake-up attack (since the invincibility frames on both is a fair bit shorter now) If off the ground hits do connect, though, they do still add to the combo meter and as such are affected damage scaling.
Once an attack lifts someone off the ground, you can basically do whatever with them. You can even do relaunch combos off OTG hits, but it require proper prediction and usually needs well timed resetting to accomplish..
Although Tech chasing was a bit similar, they focus more on prediction. Off the ground combos focus on using landing attacks before the opponent makes an action. They're generally hard to land at high level play, but very effective against low-level opponents.
Most attacks you can tech chase off of you can also OTG after. You can also do OTG combos after tripping someone. Any attack that knocks down (like The Dog's Down Strong) is very easy to OTG after, although very few characters have moves like this.
Some attacks land untechable knockdowns and allowed for guaranteed combos into and from OTG attacks since the foe can't role away or tech immediately. Examples of moves that work like this are Ryu's Jodan Sokutou Geri and Mach Rider's Electric Grapple.