Most of you probably know how to upgrade or will disagree with some things, but I'll post it anyway since it works for me.
Situation 1: You only have 25% oases in proximity of your 15c
# 13 level 4 fields - 2 level 5 fields
# Grain mill level 1
# 2 level 5 fields - 13 level 6 fields
# Grain mill level 2
# 15 level 6 fields
# Grain mill level 3
# 10 level 7 fields - 4 level 8 fields - 1 level 10 field
# Grain mill level 5 - bakery level 2
# 2 level 7 fields - 12 level 8 fields - 1 level 10 field
# Bakery level 3
# 9 level 8 fields - 5 level 9 fields - 1 level 10 field
# Hero Mansion level 10 - first oasis
# 14 level 9 fields - 1 level 10 field
# Bakery level 4
# 14 level 10 fields - 1 level 11 field
# Bakery level 5
# 15 level 11 fields
# Hero Mansion level 15 - second oasis
# 4 level 13 fields - 11 level 14 fields
# Hero Mansion level 20 - third oasis
# Continue building the crop fields
Situation 2: You have one 50% oasis and two 25% oases
# 13 level 4 fields - 2 level 5 fields
# Grain mill level 1
# 2 level 5 fields - 13 level 6 fields
# Grain mill level 2
# 15 level 6 fields
# Grain mill level 3
# 4 level 6 fields - 11 level 7 fields
# Hero Mansion level 10 - first oasis
# 10 level 7 fields - 4 level 8 fields - 1 level 10 field
# Grain mill level 5 - bakery level 2
# 2 level 7 fields - 12 level 8 fields - 1 level 10 field
# Bakery level 3
# 14 level 9 fields - 1 level 10 field
# Bakery level 4
# 14 level 10 fields - 1 level 11 field
# Bakery level 5
# 15 level 11 fields
# Hero Mansion level 15 - second oasis
# 15 level 14 fields
# Hero Mansion level 20 - third oasis
# Continue building the crop fields
Situation 3: You have two 50% oases and one 25% oasis
# 13 level 4 fields - 2 level 5 fields
# Grain mill level 1
# 2 level 5 fields - 13 level 6 fields
# Grain mill level 2
# 15 level 6 fields
# Grain mill level 3
# 4 level 6 fields - 11 level 7 fields
# Hero Mansion level 10 - first oasis
# 10 level 7 fields - 4 level 8 fields - 1 level 10 field
# Grain mill level 5 - bakery level 2
# 2 level 7 fields - 12 level 8 fields - 1 level 10 field
# Bakery level 3
# 14 level 9 fields - 1 level 10 field
# Bakery level 4
# 15 level 10 fields
# Hero Mansion level 15 - second oasis
# 14 level 10 fields - 1 level 11 field
# Bakery level 5
# 15 level 14 fields
# Hero Mansion level 20 - third oasis
# Continue building the crop fields
Situation 4: You only have 50% oases in proximity of your 15c
# 13 level 4 fields - 2 level 5 fields
# Grain mill level 1
# 2 level 5 fields - 13 level 6 fields
# Grain mill level 2
# 15 level 6 fields
# Grain mill level 3
# 4 level 6 fields - 11 level 7 fields
# Hero Mansion level 10 - first oasis
# 10 level 7 fields - 4 level 8 fields - 1 level 10 field
# Grain mill level 5 - bakery level 2
# 2 level 7 fields - 12 level 8 fields - 1 level 10 field
# Bakery level 3
# 14 level 9 fields - 1 level 10 field
# Bakery level 4
# 15 level 10 fields
# Hero Mansion level 15 - second oasis
# 14 level 10 fields - 1 level 11 field
# Bakery level 5
# 15 level 13 fields
# Hero Mansion level 20 - third oasis
# Continue building the crop fields
Basically the Guide I've been using lately and it always went pretty well for me. Though with Egyptians it's a bit of a mystery when Waterworks upgrades would give an optimal increase, but for other tribes it should work just fine.
Things achieved with this strat:
Teutons, si2 - All lvl 18 @day 75 *(first attempt)
Romans, si1 - All fields lvl 18 @day 65 *
Egyptians, com29 - All fields lvl 18 @day 50 *
* - No farming or resource import included, achieved purely from the capitals own production with ~10% production export.