Previous Versions of Omega Flowey can be found here. Previous Versions of the Load spell can be found here.
Omega Flowey was added in Beta 5.2 as the fourth Determinationcard. Like all other current Determination cards, Omega Flowey has a special ability unique to him.
As of Beta 9.1, Omega Flowey will save the board and add a Load spell to your deck. This means that when you use the spell, the cards on the board will be replaced by the ones when Omega Flowey was placed. This also means that if Omega dies, using the spell will bring him back. The spell costs 15G as well.
This card can be useful during the entire match. You can try using it as soon as possible so when you load, it goes back to near the start of the game, making the enemy lose their more valuable cards. It's more beneficial using mid-game, when you have some money to spare. If you have enough money, you can first use cards like Asgore or Mettaton NEO to wipe the board, followed by Omega in order to not save any enemy monsters.
Omega Flowey is a Determination card, and as with all cards of this rarity, it can't be Silenced and is banned from Ranked since Beta 9.3. It can also only be obtained through:
Normal Packs (with great luck);
Final Packs (which you get 1 of at LV 200);
Special Events (mostly on Twitter);
Crafting with DT Fragments. You can get DT Fragments by:
Buying one for 90 Loyalty Points;
Reaching Diamond III or higher in Ranked. (Diamond = 1; Master = 1 or 2)