No reservations are needed as far as I know. The rail planner app is often wrong about mandatory reservations in that region.

What is your travel date and departure time? Only then people can check for sure.

We are going on Tuesday 8th August. Travel time 9:30 

Can we get on and chance that we get a seat. Train is coming from Zagreb so hopefully a lot of people will be getting off here in Ljubjana

I suppose you mean the train at 9:23.

I can book a seat for €3 on tickets.oebb.at in 2nd, or free of charge in 1st class.

The question is though whether that train will run, will run with a delay, is (partially) replaced by a bus or doesn't run at all in Slovenia, given the recent floodings.

It does not let me book a seat. Have you the destination as Frankfurt or schwarzach st vein. Need seat on the Ljubjana to schwarzach st vein train

I tried what you said: Ljubljana to Schwarzach-St Veith. I could get to the payment page and saw a carriage and seat number.

But as of now, it looks like there are only 1st class seats left.

We enjoying using the oass, and still in travelling with eurail global pass,  and we seldom do set reservations accept compulsory. We do forecast from eurail planner,  and go to railway station check with the schedule screen for actual time, we often do amendments on the train timetables depending our convenience.