is it still drivable?

Hi, I park at the bus parking lot, have a 96 Explorer with 125,000 miles. For two nights, it was super cold out, and I thought that was why the check guage light was on, and the car was knocking, like it wasn't warmed up. It was less than 5 miles home, and both nights after about 3 miles, the sound went away and the light went off. I thought it was mostly because of the super cold temps. Yesterday morning I checked antifreeze, was okay, but definitely was low on oil. took it in, got an oil change, he said when he got under it the oil plug and oil filter weren't totally tight, and must have had a very slow leak there. He did put that on the receipt, and I definitely will call the repair place tomorrow that I think did the last oil change. consequently he said the knocking won't go away, and I should think about getting a different vehicle, cuz eventually the engine will be shot. I forgot to ask him if it's drivable in the meantime? I did drive it yesterday to the store, and the knocking is still there, but doesn't seem as loud, but gets louder if at an idle. I will call him first thing in the morning, I'm getting laid off the end of this month, so I might have no choice but to get the engine fixed. Guess I'll also find out then would anything else have been ruined. Thanks for any help!