Did the souls in the OT have the excuse of ignorance?
Some did yes, obviously some were utterly wicked, but yes, before Christ all the races with exception to Israel lost the knowledge of God. We can actually sort of see this in the Bible as in the early parts of Genesis there is an almost universal recognition of God, and that makes logical sense since God saved Noah and his sons whom are the fathers of everyone alive today, but by the time we get to Abraham there are very few that retain the knowledge of God, and by the time of Moses it's pretty much just Israel and those that sojourn with them.
The good news is that with Christ's birth, crucifixion, and resurrection then the knowledge of God began to spread to the other races via the apostles (this forms a large plot point of book of Acts and the backdrop for Paul's letters) so that the world may be given the knowledge of God again so that it has no excuses. On the other hand though this means the world will be even more wicked on this side of history because they will simply reject God and his good ways, thus the extreme curses of the book of Revelation that are promised for them culminating ultimately in the total destruction of creation. Though one good turn deserving another, this also means that those that believe in Jesus and his good ways will indeed inherit the promise of God of eternal life.
This passage comes to my mind immediately, and I think is fairly decent in supporting this notion:
Acts 17:24-31
24
God that made the world and all things therein, seeing that he is Lord of heaven and earth, dwelleth not in temples made with hands;
25 Neither is worshipped with men's hands, as though he needed any thing, seeing he giveth to all life, and breath, and all things;
26
And hath made of one blood all nations of men for to dwell on all the face of the earth, and hath determined the times before appointed, and the bounds of their habitation;
27
That they should seek the Lord, if haply they might feel after him, and find him, though he be not far from every one of us:
28 For in him we live, and move, and have our being; as certain also of your own poets have said, For we are also his offspring.
29 Forasmuch then as we are the offspring of God, we ought not to think that the Godhead is like unto gold, or silver, or stone, graven by art and man's device.
30
And the times of this ignorance God winked at; but now commandeth all men every where to repent:
31
Because he hath appointed a day, in the which he will judge the world in righteousness by that man whom he hath ordained; whereof he hath given assurance unto all men, in that he hath raised him from the dead.