Clearance from Transformer Oil Containment Pit to Fence

In the US, you should look at the National Electrical Safety Code (NESC) for clearances from exposed live (energized) parts such as the transformer bushings and connecting conductors to grounded facilities such as fence. That minimum clearance depends on voltage. You did say that it is a padmount, so I'm not sure that this would apply.

Clearances for the oil containment do not depend on this. Design of the containment needs to take into account 2 main things: maximum "footprint" of the transformer, so that any leaks dripping off end up in the containment, and and maximum volume of oil required to be contained. The oil containment is not energized (usually concrete), so you don't need to deal with it in clearances unless there is something external that is grounded (railing or other metallic structure). Then you apply the NESC just as if it were a fence or other such.

I believe there are other IEEE standards that give requirements for clearance from a transformer to a building or other structure that you have to protect in the case of fire, but I don't know which standard # it is (there are so many).

Hmm, so it sounds like from the statement above, that technically you could put the security fencing inside the oil containment pit if the pit was wide enough for adequate clearance from the energized systems. Usually in other systems we are required by the standards of practice to maintain security fencing at least a distance further than a person might be able to reach through the fence, but there are other issues to consider also, safe ingress/egress and mobility within the fenceline for o&m. Generally, it is offset 2 to 3 feet, around the perimeter is good. Allows movement around the facility and separation from potential outside access.