the performance department of the operator does all the calculations prior starting service in that airport
Normally the ops eng folk provide OEI data. The crew flight operating manual (by whatever name known) should have AEO data lifted from the OEM AFM.
where are those opt performance data coming from? because I don't know where to look for to find these performance values on paper.
The OEM AFM/FCOM should have AEO data.
table named "TakeOff Field/Climb limit weights" but does not specify which gradients are assured within those weights if any.
These will be the WAT-limiting weights. Gradients depend on the number of engines and are specified in the Design Standards .. eg the second segment WAT limit for a twin is 2.4% gross.
pilots can get a concrete value of their 2ENG climb gradients before departure
A few years since I crawled over a 737 FCOM/AFM .. plenty of data there for folk to read.