  NLO QCD corrections to the process $pp \rightarrow e^+\nu_e \mu^-
\bar{\nu}_{\mu} \tau^+\tau^- b \bar{b}+X$ are presented with the full off-shell
effects included. The calculation includes all resonant and non-resonant
Feynman diagrams, photon and $Z$-gauge boson contributions, as well as
interference effects, all incorporated at the matrix element level. Furthermore
all heavy intermediate particles are described via Breit-Wigner propagators.
Theoretical uncertainties related to the scale variation and choice of PDF sets
were investigated at the integrated and differential level. The modelling is
studied by a direct comparison of the full off-shell process to the NWA, where
tops, $Z$- and $W$-gauge bosons are on-shell. Moreover, motivated by
experimental cuts, we also investigate the impact of imposing a window cut
around the mass of the $Z$-gauge boson on the full off-shell predictions.
