O'Neill has revealed that he spoke with Lambert about the demands that the job would make on Keane's time before allowing the move.
"My view is if it doesn't impinge on the job here with Ireland, then in actual fact it will end up being good for him," O'Neill told the Irish Independent.
"I know that if we couldn't have made it work, he wouldn't have taken the job in the first place.
"Villa have agreed to give him all that time with us and therefore I don't really have a problem with it. I spoke with Paul to see how he felt it was going to work."
Keane has previously managed Sunderland and Ipswich Town.