Function oid_column 

pub fn oid_column(
    entries: &[IndexEntry],
    order: &[u32],
    oid_len: usize,
) -> FixedSizeBinaryArray

The oid column alone. Every arm stores it, and no arm packs it into the payload: the oid is the key the stree resolves against, not one of the facts a resolved lookup fetches, so putting it next to the payload would widen every payload read for a field nobody reads at lookup time.