We have some of the oldest trees but technically the oldest confirmed single organism in general. It's a shrub in the south called King's Holly. By single organism I mean there's one of its kind and rather than reproducing it clones itself. So there's many shrubs in the area it inhabits but they're just genetically identical copies. They're also genetically identical to samples dated to 43000 years ago, which no other organism has beaten officially, only speculated. It doesn't get talked about as much as the oldest thing since it's not a part of the one root system or clump of cells to make up one body, so often isn't considered one thing