ERA5 Top of Atmosphere Outgoing Longwave Radiation (TOA OLR)

The convention for radiation quantities in ERA5 is that thermal radiation is referred to as 'long-wave radiation emitted and absorbed by the surface'.

Solar radiation is referred to 'short-wave radiation emitted by the Sun, then scattered, absorbed or transmitted by the atmosphere and reflected or absorbed by the surface'

As a result,  Top of Atmosphere Outgoing Outgoing Longwave Radiation corresponds to top thermal radiation in ERA5 dataset.

Robin Hogan' of ECMWF has produced a document on radiation that has more details: Radiation Quantities in the ECMWF model and MARS.