Not sure what you have, but try
string[] arr = oRngListRow.Value2.Cast<object>()
.Select(x => x == null ? null : x.ToString()).ToArray();
close to what you had in your own comment.
Since C# 6.0 (from 2015), this can also be written:
string[] arr = oRngListRow.Value2.Cast<object>()
.Select(x => x?.ToString()).ToArray();
If you want an empty string instead, in the case where the entry is a null object, you can use string interpolation, also since C# 6.0:
string[] arr = oRngListRow.Value2.Cast<object>()
.Select(x => $"{x}").ToArray();