Softening caked up Tru Oil isn't practical, as noted above, it would be like trying to unbake a stale cake.
Unless the entire bottle is hardened there is usually just a skin over the top that you can poke through to get good liquid out that's underneath.
Storing it upside down as mentioned above is the best way to prevent a skin forming on the top. Another way that works quite well is to fill the empty space in the bottle with BB's until the liquid is forced right there to the very top of the bottle.
Tru Oil is a witches brew of boiled linseed oil, Japan Drier, other hardening and glossing agents. I've used a boat load of it, and wasted a lot of time on stocks controlling what type look I needed for each.
I've had better luck with just Boiled Linseed Oil and adding my own dose of Japan Drier to it. Heated on an electric stove eye in a small stainless steel measuring cup, then liberally brushing on the wood with a cheap bristle brush and hung in a warm environment for about 20 to 30 minutes, or until it just begins to tack up. Wipe down dry with a cheap cotton wash cloth and hung over night to cure, then buff down with a 000 or 0000 scotch brite pad. Multiple coats in that manner give a better finish build up than Tru Oil.
For a final finish I use axe wax for a warm glow and if I want glossy I use renaissance wax.