After sitting for a few weeks, all the oil drained out of the tank back into the engine. Question is, can I start the engine as usual or do I need to somehow pump the oil back (crank the engine with kill switch off?) into the oil tank?

ALL the oil? It might be a good idea to pull the plugs to make it crank easier and spin it with the starter till there's some oil in the tank for the pump to use on the pressure side. Before doing that tho', put it in gear, (high) and roll it to make sure there's no hydraulic lock. It sounds like your check valve hung up on ya. If the scavenge side of your pump is working , then start it and make sure you are getting oil pressure and go through a full warm up cycle. Ride it if you can. When you shut it down, check the oil. Then check it in 30 minutes. Then in an hour. Two hours. Over night. It may be fine now, but if not, you'll know how fast it's returning to the sump. Are you sure that's where it is? I ALWAYS check my oil before AND after a ride because a hose can spring a leak and somehow go unnoticed, and pump all yer oil on the ground. Did you recently change the type of oil you use? Is it due for a change? Overdue?

After sitting for a few weeks, all the oil drained out of the tank back into the engine. Question is, can I start the engine as usual or do I need to somehow pump the oil back (crank the engine with kill switch off?) into the oil tank?
pretty common...the longer you let them sit the more oil will drain out past that check valve...there is even a note about this in the owner's manual I believe...shouldn't be too big a deal to start it back up...I look at it as a sign that the bike isn't being ridden enough :D

Mine does it and I just start her up and away she goes. It pumps into the oil tank really fast.

There was just an blurb about this in American Iron Magazine. They mentioned that this is common in the Sportsters and Softails and there is nothing to worry about.

Yeah, before I bought my bike, it was sitting for 5 years. There was just a puddle of oil in the tank. I added about a half quart before I started it just to be sure. Fired right up and the oil came back. No biggie. I just heard that you have to watch the crank case vent as oil MIGHT shoot out of there. But mine didn't.