You should set the viewer style to Custom Lemmings, not ONML. In CustLemm, the original styles have the IDs 0 thru 4 and the ONML styles 5 thru 8, while in ONML viewer style the ONML styles have the IDs 0 thru 3. Therefore selecting ONML for your original levels will display them with the wrong style set.
So, compile the CustLemm styles (if you don't already have), and then simply select it as viewer style.