It is worth noting that the obstacle assessment surface (OAS) method of calculating DA/DH for an ILS works completely differently to the obstacle clearance altitude (OCA) calculation used in a 2-D approach.
With the OCA, no obstacles may infringe the surfaces: however, with the OAS you expect obstacles to infringe the various surfaces of the OAS (ignoring the missed approach surfaces for now!). The highest obstacle that infringes the surfaces is then used to calculate the DA/DH by adding the relevant height loss margin for the applicable aircraft category, to the elevation/height of the obstacle.
The issue now is that if the controlling obstacle is early in the final approach segment then it must be high (otherwise it would not infringe) and therefore the DA/DH will be unacceptably high. Options at this point include increasing the glidepath angle above 3 degrees (favoured choice) or (if the obstacle is close to the final approach point) PANS-OPS currently allows the designer to introduce a final approach fix which, if the obstacle is below a 15% plane starting at the FAF earliest fix tolerance area at the intermediate segment OCA minus the intermediate MOC then it can be ignored. That is great; however, now that you have a FAF you can't start your descent from the intermediate OCA unless you are at the final approach fix (otherwise you reduce your clearance from the ignored obstacle!) so on an ISA + day, you will not be able to intercept the glideslope from below!
Typically the designer would use the 'Collision Risk Model' (CRM) to analyse the obstacle environment but that involves the casting of bones and use of magic!!
Cheers
TeeS