Assuming you have your horn tuned "at home" to A=440 (or whatever pitch your ensemble prefers), it is often difficult to find one's pitch in a large ensemble.
When the oboist gives an "A" and you compare that to your written e' or
e", you have built-in problems. The e' at the bottom of the staff is
the 5th harmonic, which tends to be "flat" in equal temperament. Fourth
space e" is the 8th harmonic, which should be "in tune" but you have
likely pulled that second valve slide on the B-flat horn a bit to bring
down the sharp 6th harmonic (b'). If you use either of these flat
fingering, and then push in your main tuning slide, you will be
generally sharp on the rest of the instrument. So, what to do?
Here is a tip: try 1+3 e" on the B-flat horn. Wait - aren't 1+3
combinations sharp? Well, yes, but that is the 10th harmonic of that
series, which is flat, so they cancel each other out and you have a
fingering that should be pretty well in tune. So try toggling between
the written b' (slightly sharp but that should make a beat-free fourth
with the oboe a') and 1+3 e".