My understanding is that Church Slavonic is nearly indecipherable to someone who knows modern Russian, which would make Old English the correct comparison. However, I am not fluent in Russian and am relying on others' characterization.
Note, however, that OCS does not have an ancestral relationship to Russian - it is a Southern Slavic language, most similar to ancestors of today's Bulgarian. Old Russian texts (such as The Tale of Igor) are written in a different language - Old Russian (which existed long before the split into Russian, Ukrainian and so on, so the name "Old East Slavic" is sometimes used).