/usr/share/gromacs/top/oplsaa.ff/ is in gromacs-data 2018.1-1.
This file is owned by root:root, with mode 0o755.
..
/usr/share/gromacs/top/oplsaa.ff/1propanol.itp
/usr/share/gromacs/top/oplsaa.ff/aminoacids.c.tdb
/usr/share/gromacs/top/oplsaa.ff/aminoacids.hdb
/usr/share/gromacs/top/oplsaa.ff/aminoacids.n.tdb
/usr/share/gromacs/top/oplsaa.ff/aminoacids.r2b
/usr/share/gromacs/top/oplsaa.ff/aminoacids.rtp
/usr/share/gromacs/top/oplsaa.ff/aminoacids.vsd
/usr/share/gromacs/top/oplsaa.ff/atomname2type.n2t
/usr/share/gromacs/top/oplsaa.ff/atomtypes.atp
/usr/share/gromacs/top/oplsaa.ff/ethanol.itp
/usr/share/gromacs/top/oplsaa.ff/ffbonded.itp
/usr/share/gromacs/top/oplsaa.ff/ffnonbonded.itp
/usr/share/gromacs/top/oplsaa.ff/forcefield.doc
/usr/share/gromacs/top/oplsaa.ff/forcefield.itp
/usr/share/gromacs/top/oplsaa.ff/gbsa.itp
/usr/share/gromacs/top/oplsaa.ff/ions.itp
/usr/share/gromacs/top/oplsaa.ff/methanol.itp
/usr/share/gromacs/top/oplsaa.ff/spc.itp
/usr/share/gromacs/top/oplsaa.ff/spce.itp
/usr/share/gromacs/top/oplsaa.ff/tip3p.itp
/usr/share/gromacs/top/oplsaa.ff/tip4p.itp
/usr/share/gromacs/top/oplsaa.ff/tip4pew.itp
/usr/share/gromacs/top/oplsaa.ff/tip5p.itp
/usr/share/gromacs/top/oplsaa.ff/tip5pe.itp
/usr/share/gromacs/top/oplsaa.ff/watermodels.dat