Ol' here is old worn soft, not about age. Big ol' truck means a notably large truck. Adds rhythm and size without fancy adjectives.
How folks use it
“He drove up in a big ol' Ford.”
“That is a big ol' problem you brought home.”
Origin and etymology
Southern and African American English habit of slipping ol' into size talk. Old does not mean elderly here; it is a flavoring word. Common in casual speech.
Meaning
How folks use it
Origin and etymology