Staircase made from OSB base instead of concrete - More cost-effective?

It is probably the oak that is load-bearing and the OSB the upper covering. OSB is chipboard; under normal circumstances, you cannot build a staircase from it. Are you sure that such a staircase will be cheaper than a concrete staircase?

Well, cheaper doesn’t always have to be worse...

I wasn’t aiming for an objective "worse" – from a buyer’s perspective, a minus point in value is enough if something simply looks a bit less valuable. A concrete staircase is the "recognized state of the art," a storage room with a walkable top is not.

As I said: if the stair builder came up with an alternative, I would consider it – but if the stair layer says, oh, let me be the stair builder too, I am skeptical. Then I think that incorporating this additional manufacturing depth benefits him more than the design change benefits me.

Our concrete staircase cost a net of 1,700 EUR. Since it is to remain untreated, our carpenter made a temporary protection for it out of OSB. Cost: 400 EUR

And it was just about making a COVERING without any special requirements for load-bearing capacity or anything else.

You would have to add the oak top layers on top anyway.
So what's supposed to get cheaper there?

Besides, in my opinion, the staircase will then be cheap—but also cheap-looking!