@RenaisanceMan I’m not sure how familiar you are with the rules of Hi Lo, so apologies if my answer goes back to the basics a bit, but it’s just to make sure your question is answered.
In Omaha, the first rule is that you strictly need 2 cards from your hand and 3 from the board to make a hand. The high and the low both need to be made of 2 cards from your hole cards and 3 from the board. You cannot use only one of your cards, or 3, or all 4. Strictly 2.
And all hands have a high, but not all hands have a low. To have a low, the board should have at least 3 different cards from A to 8. The lowest 3 cards from the board + the lowest 2 cards from your hole cards (all between A and 8 and all different) would make the low.
You are saying that your opponent won the whole pot with just the Hi. As we can’t see which hand you’re talking about, I’m assuming there was no Lo at all in that hand. So if the board showed only 2 low cards, 7 & 5, and you had 4, 3 and A, this cannot count as a hand because it’s 2 cards from the board. You HAVE to use 3 from the board and 2 from your hand. Even if the board was showing Q,7,5,4,3 and you had the ace but all other 3 cards are higher than 8, you still don’t have a Lo because you strictly need 2 cards from your hole cards.
If that wasn’t the issue, and you really had 7,5,4,3,A made up of 2 hole cards and 3 community cards, and you still didn’t win part of the pot, it can only mean that someone else had a lower Lo. The same player could have both the best Hi and the best Lo and win the whole pot. To know which Lo is the lowest, you have to look at the hands from highest to lowest card and compare.
For example 6,5,4,3,A beats 7,5,4,3,A because 6 is lower than 7.
7,5,4,2,A beats 7,5,4,3,A because 2 is lower than 3.
5,4,3,2,A beats them all as it’s the lowest possible.
I hope this helps, if not please do post the hand in question and we’ll all try to explain what happened
All the best.