It would be really difficult to study OCS without some knowledge of a Slavic language--the most useful being Bulgarian, though Russian is also extremely helpful. (When I studied OCS as a very beginner Russian student, everyone else in the class was a graduate student with knowledge of Bulgarian or Russian. It was really, really difficult to keep up...)
The standard textbook, it seems, is Horace Lunt's Old Church Slavonic, though I found it absolutely impossible to use given its liberal use of transliteration and really wonky linguistic terminology. If you know Russian, you can use the Soviet-era textbook by Khaburgaev (Г.А. Хабургаев), which is far more useful. I've been meaning to buy Fr. Alypy's book, but it's sort of low on the priorities right now.
Know that OCS is a very, very difficult language, but it makes sense after some study. The most difficult parts are the abbreviations, which make sense if you are least familiar with the words and know they're coming.