I was recently reading that Cessna introduced 'Skid Rails' to the underside of some O-2's, to minimise the damage when a/c arrived back at base with battle damage to undercarriage.
Here is a link to one such O-2, clearly showing the 'rails' fitted. This O-2 is ex 68-10989, c/n 337-M0265.
http://www.airliners.net/photo/Chile...5d0fee2393dfe4