Olympus OMD EM-5 body + the 45mm as my only lens (is it possible?)

Sure, but I'd have minimum two primes, the 45 and the Sigma 19mm.

I do not currently have the 45 1.8, my first copy was dissapointing in decentering.  I actually feel like the system is incomplete as I only have the 19mm and the 40-150 right now.  40-150 is great with enough light however, but I would love to have a 45mm 1.8 that was well-centered (like the one on photozone).