Ohlendorf (1-0) was called up from Class AAA Syracuse to make his season debut and his first appearance with the Nationals.
He had not pitched in a major league game since last August, when he was with San Diego.
Ohlendorf threw 89 pitches in six innings, allowing two Colorado hits and a run while striking out two.
Jorge De La Rosa (7-4) gave up three runs on four hits in 4 1/3 to take the loss.
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