An oven which is exclusively used for meat should not be used to prepare uncovered Parve foods that will be served at a dairy meal (and vice versa for preparing Parve foods in a dairy oven for use at a meat meal) (See Shulchan Aruch Yoreh Deah Siman 92:8, 108:1 and 2 and Rama Siman 93:1 and 95:5. Even though regarding cooking something Parve in a meat or dairy pot that wasn’t used in 24 hours, according to the Shulchan Aruch it is more lenient, or even permissible L’Chatchilah, it is different in an oven as there are actual particles of food left on the walls etc.)
If a Parve food was in fact baked in such an oven, uncovered, if the oven was not cleaned out prior to this baking, the food assumes the status of the oven and may not be eaten with the opposite food class, and if the oven was dairy, may even require waiting 6 hours [or whatever time one usually waits after meat] before eating the baked item. (See Shulchan Aruch Yoreh Deah Siman 97:3 and 108:1. See also Kovetz M’Bais Leivi Vol. 3 page 21 Os 3. See also Shach Yoreh Deah 108:11 and Darchei Teshuva 108:41)
If an uncovered parve item was cooked/baked in a clean meat or dairy oven that was used within 24 hours, even though the oven was cleaned, it should still not be eaten together with a food of the opposite class, but may be eaten immediately prior or afterwards.
However, some Poskim are lenient with this and maintain that as long as the oven was cleaned and turned onto the highest setting for a while (half hour or an hour) then the Parve items may be eaten with a food of the opposite class.
This is especially relied upon at times of necessity such as Erev Shabbos or when one has guests. (See Shu”t Minchas Yitzchak Vol. 5 Siman 20)