Depending on the saw the chain oiler works one of two ways.
Very low end saws (like the one I carry) may be "Drip oilers" suffice to say they leak, however you have already told me yours is not that type (It does not leak a drop at home)
Beter ones it's a engine driven (or gear driven or cam operated) pump. These should not leak but heat can cause pressure to build in the oil chamber, Likewise altitude changes induce pressure relative to atmospheric
And bouncing around can operate the pump.
Id' start with benting the cap to equalize pressures
I would also bucket the saw (set it in something where the leak does not make you as upset) when not sawing.