For severe rust, disassemble and use WD-40, let it soak for 24 hours and clean as normal. DO not use WD-40 as a lubricant, it may gum up the firearm. (There are a lot of rust removal products out there, all work pretty well, wd-40 works great for me)
For minor rust spots I have used gun cleaner and green scrubbing pad. Some folks will use very fine (0000) steel wool, and it will work but it can also damage the bluing, green/blue scrubbers will not.
If the guns are in bad shape, lot of rust or broken parts, you will need a gunsmith to look at them and pronounce them usable or wall hangers.
Just an FYI folks, bluing IS rust, and any method of removing rust, chemical, abrasive, electrical, will eventually remove it if carried to extremes, so start with a non visible area, go slow, check your results frequently before diving into the entire gun.
When you apply oil to rust that will temporarily block oxygen in the air from reacting with the iron in the metal. Usually a better and more permanent approach is to first apply dilute phosphoric acid. That's the basic ingredient in most all "rust conversion" products, such as Naval Jelly, Jasco Metal Prep (or Etch) etc. The phosphoric acid converts the rust (iron oxide) into iron phosphate, which inherently tends to stop rust from forming. An advantage is that rust in tiny pockets or pits is converted too.
Oil alone is a temporary barrier to prevent oxygen in the air from reaching the iron.
Depending on what they are, "Old guns" may be collectable, and how you treat the rust could greatly impact their value. For surface rust, start with oil and a rag. You will probably be surprised how much of the rust comes off.
If it needs more, move to bronze wool. Keep moving up the ladder of how aggressive(read: destructive) you go. The more original finish you can preserve the better, and the less value they can lose.