Nobody's suggesting that they could beat the Warriors, but if they merely take last season's model of Westbrook-everything and give him spot up shooters of the quality of PG and Melo, that's a massive upgrade on Oladipo, Grant and Roberson. Let alone the upgrade when Westbrook isn't on the court (which, as shown by the plus minus vs Houston, is where OKC really lost out last year). If they can get Melo going back to the basket and passing out of double teams to a driving Westbrook and PG + Roberson guarding the opposition's best player, they can definitely take Houston, nullify Kawhi, and give the Warriors a run for their money. I'm not a Melo fan or a Westbrook fan but there's only one team in the NBA with a better big three, because of Isaiah's uncertain status. Melo is definitely good enough to be the third piece on a title winning team.
Will they mesh, will Westbrook cede late game shots to Melo, will PG be willing to take on a team role in the prime of his career, will Melo be able to play defence, will Felton, Roberson and Patterson give quality contributions, who knows. But from my POV, if they merely do everything the same as last year and drastically underutilise both Melo and PG while Westbrook is on the court, they will still be more than competitive with Houston and SA.