Most, about 95% of outdoor wood boiler / heater systems, DO NOT REQUIRE O2 Barrier PEX for the insulated underground pipe. They are an OPEN system. Brands include Central Boiler, HeatMaster, Hawken, Natures Comfort, Earth Outdoor, Hardy, Polar, Portage & Main, WoodMaster, Crown Royal, Ridgewood, Tenessee, and many others
HINT: Look for a vent on the top of your boiler, this is a clue #1 to an OPEN system. An OPEN system does NOT require O2 Barrier PEX,
O2 molecules are smaller than the polyethylene molecules in PEX pipe and so Oxygen can actually pass through the PEX into system water. This can be harmful to "CLOSED" type systems. That is why some require O2 Barrier PEX
Once you decide about Non-Barrier or Barrier PEX proceed to Step Two and consider what size PEX to use. You can always call for help deciding.