You are right, Oa01_radiance is wrong, totally wrong. The NDVI operator, if no band is specified tries to find the band by itself.
As red signal it searches for a band between 600nm and 665nm and for the NIR signal it searches for a band in the range of 800nm and 900nm. For OLCI it finds band 7 first and for Sentinel-2 band 4 for red signal.
But this is only the default behaviour, you can select the bands you would like to use.