Zoso, there is a few reasons for an oil warning light to be on in your truck, oil could be low, oil sending unit faulty, oil pump could be faulty, clogged oil passages, worn internal engine bearings, etc. If you do not presently have a mechanical gage to read oil pressure, it would be a good idea to hook one up to see where pressure is at to help diagnose problem or take into a good shop and have them do it, diagnose and estimate repair.
Now if you are talking about a change/service engine oil light found in later models, after changing oil the light must be reset turn on ignition to on with out engine on, press the gas pedal down 3 times, holding pedal down on third press till oil indicator displays oil life reset.
Hope this helps, good luck.
https://www.yourmechanic.com/article/how-do-you-know-if-the-oil-pressure-gauge-is-bad