We study chemisorbed configurations of C3H6O2 on the extended H:Si(100) surface, through first-principles density-functional calculations in a supercell approach. We demonstrate that oxygen-bonded organic monolayers on this silicon substrate is thermodynamically very stable, and comparing several Si-O-C and Si-C linked configurations, we find that the doubly-O-bonded configuration is favored and should lead to ordered SAMs. We find, moreover, that the Si-O-C bridge in this case does not block charge transfer from surface to molecule.