I had one of the oil cooler lines replaced, and now my heater blows cold air

If we are talking about an oil cooler line - they do not influence heater operation. However, in replacing the lines, you access the cooling system and drain it. So, it's likely that there is an air pocket in the cooling system still that is preventing you from getting heat. Have the cooling system purged of air and recheck the heat. Use this link to find some shops near you that can help with your concern: www.repairpal.com/repair-shops