Twice a year, all the Hermes boutique SM's go to Paris to place their orders. They are told at that time which colors/leathers will be available. If they have promised a customer a PO, they will place it at that time. Basically, they are ordering something they most likely would have already ordered for their boutique, but it will be offered to a particular client when it arrives. PO's are limited to what is available that season.
A SO, is a true custom order. There may be some limitations (fewer ones for VVVIP customers) but this is when contrast trim/lining etc are available. Boutiques are limited in the number of SO's they may make in a year, so they are typically only offered to good customers of long-standing (not always true these days). One is later notified if the SO has been accepted by Paris - they may think what you want is not appropriate for their bag and may refuse it or suggest changes. I believe these are NOT done at the same time as the PO's are made (that is in Feb/July if I remember correctly).
A PO usually arrives within 6-9 months; SO's can take multiple years, depending on the leather/skin/trim involved.
One does not make any payment at the time of either order, and must pay whatever the price is when it arrives. With a SO in an exotic, this can be a substantial amount more than the price when it was ordered.
I think ostrich is available in a 35 but I'm sure others will know for sure.