This is the 1339th most frequent Dutch word.
oudste
"Oldest."
Here, "oudste" is used as a superlative adjective to describe the tree as the oldest among others.
De oudste boom in het park is wel honderd jaar oud.
The oldest tree in the park is over one hundred years old.
In this case, "oudste" functions as a superlative adjective indicating relative age among the siblings.
Zij is de oudste van drie zussen.
She is the oldest of three sisters.
"Oudste" here is again used as a superlative adjective to indicate the comparative age of the documents in the archive.