This used to be a subject reserved for the charting experts and the geeks who got into the nuts and bolts of aircraft performance. Speaking as one of those in the latter group, it wasn't easy to plan a takeoff based on losing an engine and just clearing all the obstacles because most pilots were trained to use their aircraft flight manual engine out data for all conditions. It was conservative, after all. Things have changed.
— James Albright
Why have they changed? In short, using flight manual (one engine inoperative) data leaves too much performance on the table when you are probably going to be taking off with all engines operating. The key, then, is to have a plan to beat the obstacles but maybe not the instrument departure procedure when losing an engine.
This is a complicated subject, but it is something you need to pay attention to before you start flying out of airports where the mountains lie in wait. I recommend the following, in order: