OPK's can always show a faint line as there is always LH in the system. Some women will get a line that gets gradually darker leading up to O and some will get nothing at all (or a faint line) until suddenly it is positive.

OPK's can be used as a HPT but the line should only be considered + if it is really quite dark. A lighter one could be + but as there could be a line anyway it is not reliable.