Weeeeel...... That O ring sort of stops the pump from bypassing the engine and going straight back in the sump. The oil pressure light is relatively low pressure. But on the other hand you've not experienced thumping big ends, growling main bearings or rattly valve gear. I tend to think you got away with it. I presume the gasket on the filter cover was not used? That just introduces a bigger potential bypass.
There is apparently some variation in the depth of the filter housing - it's pressed in? so you may have one that's nominally tight.
Fit the new filter (and maybe the o ring?
) and see what happens. Hopefully what doesn't happen.
I rode my RD 250 for about 15 miles before suddenly realising I hadn't put any gearbox oil in. I'd just replaced the main bearing seals (2 stroke) and had split the crankcase. I bunged whatever I could get at the local garage in the box and never had an issue. 2,000 miles would have resulted in destruction though.
Cutting open the filter may show something. But if it's full of bronze or steel, you'll have had audible feedback by now. Of the crunchy, buzzing, rattling or thumping kind. Shake, rattle and graunch baby!