Bishul

If potatoes have some oil upon them is that considered a Davar Lach?

ב"ה Shalom, Without you leaving many details, my assumption is that your question is in regard to reheating the potatoes on Shabbat. If this oil is already absorbed in the potatoes , which would be very common in mashed potatoes, then we consider the potatoes as "Davar Yavesh, even if some of the oil later oozes, since this oil is not wanted. If there is oil in a liquid state and not congealed, with an amount which you made something else wet "Tofeach al mnat lehatphiach", there is a difference of opinion among the poskim if to consider it "Dvar Yavesh" since the liquid is the minority and it is insignificant, or de we consider Davar Lach. Many poskim take the more stringent opinion and consider this ":Lach" and forbid warming such a thing. Nonetheless, you can be lenient if there is really no interest in the gravy coming from cooking this food and warm it as a "Davar Yavesh." Also, if you removed the potatoes from this oil and they remain just slightly wet, it is considered negligible and they may be warmed on Shabbat. (See פסקי תשובות סי' שי"ח: לב- in regard to the criteria of what is considered Davar Lach.) All the best