I checked on OEP site and find it a bit confusing.
This transformer is described as 600ohm C.T. primary to 2400ohm C.T. secondary (see attachement) but the data sheet states 150+150 primary and 600+600 secondary :-/
Doesn't resistance added in series?
Yes, but a transformer is like a lens in that it just reflects the load on the other end. So if you have 300 ohms source on the primary, the secondary will look like 1200. But if you have 600 ohms on the primary, that is ok too and so it looks like 2400 on the secondary. But if you put 10k on the primary, it's not necessarily going to be 40k on the secondary because the frequency response will start to get wonky. You'll get peaking at HF and, as Ian pointed out, the impedance is capped at low frequencies because the inductance is a little low.
You say 125 mH + 125 mH in series gives 500mH? or is it 250mH?
I think i will never understand transformers..
No you're right it is 250mH.Actually it is 500mH because of mutual inductance as Ian says below (the inductors share the core so it's 4x and not just 2x).
How about if i reverse the transformer and use only on winding (600ohm) as primary ?
Will the THD figures be better?
Yes, you could flip it around and get a 1200:300. But then you're stepping down
4x or 12dB 2x or 6dB which is slightly counter productive for a microphone amplifier.
Also if i want to add a bypass switch around the output transformer, should the output XLR connector have pin 1 & 3 shorted when in bypass mode?
It's not really a great idea to leave the transfomer hanging on the OA and then also drive the line directly. You should use a two pole switch so that you can redirect the OA output to a coupling cap w/ drain resistor into 68R or so in series to pin 2 and then the other pole to switch pin 3 to a 68R to analog ground (this is called an "impedance balanced" output and will help reject ground noise). Pin 1 should be connected to the chassis which should not be connected to analog ground at that point. Ideally analog ground and chassis ground should be connected at one point at the power supply filter cap ground (or wherever the chassis connects to the PS ground).