  The structure and decay of $^{26}$O are investigated in a three-body
$^{24}$O+$n+n$ model suitable for studies of the long-lived (including
radioactivity timescale) states. We have found extremely strong effect of the
subbarrier configuration mixing on the decay width of true $2n$ emitters due to
core recoil and neutron-neutron final state interaction. This effect is far
exceeding analogous effect in the true $2p$ emitters. Our calculations provide
reasonably narrow boundaries for the lifetime vs.\ decay energy dependence for
the true $2n$ emission. An upper limit of $\sim 1$ keV for the decay energy of
the unbound $^{26}$O is inferred based on the recent experimental lifetime
value.
