To clarify my previous response, Oklahoma statute (see Title 21 § 1277
B) specifically excludes city, county, and state parks, recreation areas, and fairgrounds from the list of prohibited places. It is unlawful for cities, counties, or the state to prohibit permit holders from carrying firearms either openly or concealed in these areas. Most of the buildings are probably off-limits though if you're carrying.
You could still get hassled if they're unaware of the law. My advice is to contact the chief administrator of the park and find out what's going on. Maybe they're just misunderstanding the law. If they're adamant about keeping the signs up after you politely explain the statute, I'd contact the Oklahoma 2nd Amendment Association
for help getting the signs down.
However, if this is a Corps of Engineers park, it's regulated by federal law and firearms are generally prohibited.