Decredenza about 50-60% of landing are at prepared Helipads which makes for easier/safer run-ons & running take-offs when required. But the rest are whatever works, anywhere, SWAT & dive in, some are one way in, same way out.
JB-319 legal requirement is to use O2 continuously from 12,000' but we are generally start using from about 10-11,000' up
VF