Avian remains are rare from Paleogene sediments in the Great Plains region of North America, especially those that can be identified taxonomically. Here, we describe avian eggshell fragments from the Fitterer Ranch locality within the Oligocene Brule Formation of North Dakota and assign them to ootaxonomy. The specimen examined (NDGS 1934) exhibits low tubercles that form undulating surface ornamentation; 650–900-µm-thick eggshell composed of three structural layers of calcite; abrupt transition between mammillary and continuous layers; prisms visible in continuous layer; squamatic texture primarily visible in lower half of continuous layer; and mammillary-to-total shell thickness ratio of 1:2.70–2.75. Due to this unique combination of characters, we establish the new oospecies Metoolithus jacksonae, oosp. nov. This new oospecies extends both the geographic and temporal ranges of the oogenus Metoolithus and demonstrates the importance of oological research in revealing the diversity of Oligocene avifauna in this region.