Typically used in a business environment, it refers to co-workers sucking up to the boss, as in a dog humping his owner's leg. Time spent doing such is known as "OTL time". Basically, another way of saying "kissing ass".
Q:"Where's Jim?" A:"I think he is in the boss's office...probably on the leg".
Q:"Ready to head home?" A:"Naw...I gotta go down to the boss's office and get some OTL time in".
"If you want to get a raise this year, you better get on the leg as reviews are coming up here shortly"