MRO inventory sits directly upstream of MTTR: a repair can't start until the part is in hand, so a stockout on a common spare turns a 20-minute fix into a multi-day wait on a supplier. Getting MRO stocking right is a balance — too little inventory stretches MTTR, too much ties up capital in parts that sit on a shelf for years.
The usual approach is stocking to criticality: high-criticality assets with long lead-time parts justify carrying stock even if it's rarely used, because the cost of the part sitting on a shelf is trivial next to the cost of that specific asset being down for a week waiting on a supplier.