I used this plugin in QGIS2 for easy defining the area of interest using some vector polygon and then cropping the OSM data with osmctools and the poly file.
However the plugin is QGIS2 only:
This is QGIS 2.x plugin.
It exports all or selected vector polygons to poly-files used by Osmosis, Osmconvert (and probably other OSM-related tools) for cliping OpenStreetMap data.
Is there any alternative for QGIS3?