Xamarin.Forms adding items/data from a Rest API to an ObservableCollection

I've got a Xaml page that I'm using ListView on to display a list of offers. I'm trying to Get offers using a REST Web API call to display on the page using an ObservableCollection. Since async calls don't seem to work on ObservableCollection methods, I've moved that to a public async task method under the public ObservableCollection GetOffers() method. Here is my code.

public class OffersService : IOffersService
    {

        public ObservableCollection<OffersModel> GetOffers()
        {
            var offersList = new ObservableCollection<OffersModel> ();

            var offers = GetOffersAsyncOC ();

            OffersModel model = new OffersModel () {

                Id = Guid.NewGuid (),
                OfferImage = null,
                OfferType = "Dollar Off",
                Name = "Test name",
                Description = "Test Description",
                Amount = 25,
                StartDate = DateTime.Now.Date,
                EndDate = DateTime.Now.Date.AddHours (8),

            };

            offersList.Add (model);         
            return offersList;
        }

As you can see, I'm using the GetOffersAsyncOC method to get a list of offers. I'm then manually adding an offer for testing purposes because I can't seem to find a way to add the offers from the Rest API to the ObservableCollection. For testing purposes, this works, but I need to be able to work with data coming from the Rest API.

Any help is much appreciated. I've been developing for less than a year, so I may need a little more explanation than usual. Thanks!