Obscurinin A (1), an unprecedented Lycopodium alkaloid featuring an 8/6/6/6/5 ring system, was isolated from the methanol extract of Lycopodium obscurum L. This novel skeleton possessed a fused 2-methyl-9-aza-tetracyclo[7.5.1.1(4,14).0(5,15)]hexadecane core with a 2-oxazolidinone moiety. Its structure was assigned by spectroscopic and single-crystal X-ray diffraction data. A biological study showed that compound 1 exhibited significant inhibitory activity on the Ca(v)3.1 calcium channel with an IC50 value of 12.00 +/- 0.35 mu M.