land, shore, Austria
墺 centers on a piece of land at the water's edge: a shore or coastal land. The Austria meaning is a phonetic loan used in country names, not an extension of the land/shore idea.
墺 combines 土 (earth, land) with 奧, which likely contributes the sound. The character originally referred to a shore or waterside land. The use for Austria is a modern phonetic borrowing.
The left side 土 is earth or land, and the right side 奧 suggests a deep or inner area. Picture a strip of land at the edge of deep water: a shore.
For オウ, imagine standing on the shore and saying 'Oh!' as you look out over the ocean: 'Oh!' -> オウ.