Oh, I loved onesies and baby undershirts!!! They do come in long-sleeved, OP, although they may be hard to find. But I always used the short-sleeved ones in the winter, and put a long-sleeved baby outfit on top. I just wanted the extra layer of warmth over the torso, and not the arms orr legs, so the short-sleeved ones worked fine. Actually, I usually used the one piece onesie that snapped between the legs year round until my girls outgrew the biggest size.
Oh, I miss those days, especially now that they have been replaced with thong panties from Victoria's Secret!!!!