After a lot of research, I think this may be a possible OT timeline (well, at least it shouldn't contradict the original scripts).
Still needs some work, though :\
====SOURCES
http://www.imsdb.com/scripts/Star-Wars-A-New-Hope.html
http://www.imsdb.com/scripts/Star-Wars-The-Empire-Strikes-Back.html
http://www.imsdb.com/scripts/Star-Wars-Return-of-the-Jedi.html
http://originaltrilogy.com/forum/topic.cfm/Implied-starting-date-of-the-Empire-from-OT-dialogue/post/793052/#TopicPost793052
http://www.starwarsnewsnet.com/2015/10/swnn-interview-james-kahn-author-of-return-of-the-jedi-novelization.html
====TIMELINE
-25,000
The Old Republic, the first galaxy-wide government is formed;
Jedi Knights appear;
period of galactic peace and expansion begins.
AGTTSWU (A Guide to the Star Wars Universe)
-896
Yoda is born.
"When nine hundred years old you reach, look as good you will not."
AGTTSWU
-200
Chewbacca is born on Kashyyyk.
AGTTSWU
-112
C-3PO is built in a Cybot Galactica plant on the planet Affa.
George Lucas
>-60?
Obi-wan is born.
"Surely he must be dead by now" (Tarkin considers him very old)
60 BSW4: Obi-Wan Kenobi is born. (AGTTSWU)
Obi-wan becomes Yoda's apprentice
"[...]Yoda, the Jedi Master who instructed me."
-55
Anakin is born.
AGTTSWU
-48
Mon Mothma is born on Chandrila.
AGTTSWU
-?
Obi-wan serves Organa in the Clone Wars
"General Kenobi, years ago you served my father in the Clone Wars."
AGTTSWU
-37?
Anakin becomes a pilot.
"He was the best star-pilot in the galaxy"
Anakin becomes Obi-wan's apprentice.
"When I first knew him, your father was already a great pilot.
But I was amazed how strongly the Force was with him."
"I took it upon myself to train him as a Jedi."
Anakin and Obi-wan fight in the Clone Wars.
"You fought in the Clone Wars?"
"Yes, I was once a Jedi Knight the same as your father."
-35
The Clone Wars end.
AGTTSWU
-?
Anakin leaves his training
"When I left you, I was but the learner"
Anakin faces Palpatine, but is defeated.
"If you end your training now, if you choose
the quick and easy path, as Vader did,
you will become an agent of evil."
"Do not underestimate the powers of the Emperor,
or suffer your father's fate, you will."
Anakin is turned to the Dark Side.
"You father was seduced by the dark side of the Force.
[...] the good man who was your father was destroyed."
Anakin becomes a royal under the name of Darth Vader, Lord of the Sith.
"He ceased to be Anakin Skywalker and became Darth Vader."
-?
Vader faces Obi-wan. The duel leaves him dying falling into a lava pit, but he menages to survive.
George Lucas
ROTJ's novelization
-?
Last days of the Republic and the opening moments of the Empire
AGTTSWU
Vader hunts down the remaining Jedi.
"Darth Vader [...] helped the Empire hunt down and destroy
the Jedi Knights. [...] Now the Jedi are all but extinct."
Obi-wan assumes a new identity, Ben Kenobi.
"I haven't gone by the name Obi-Wan since oh, before you were born."
-18
Luke and Leia are born.
"Luke Skywalker, a farm boy with heroic aspirations
who looks much younger than his eighteen years."
AGTTSWU
Their birth is kept secret.
"To protect you both from the Emperor, you were hidden
from your father when you were born."
Luke is rised by his uncle.
Leia is initially rised by her mother, before her death.
"Leia... do you remember your mother? Your real mother?"
"Just a little bit. She died when I was very young."
Leia is adopted by the Organas
"That is the reason why your sister remains safely anonymous."
-0
"Star Wars"
====TODO
The Clone Wars begin
Palpatine is elected Chancellor