This will make a bitmap out of an oxyplot plot model.

OxyplotBitmap.cs

using System;
using OxyPlot;
using OxyPlot.Axes;
using OxyPlot.Wpf;
using System.IO;
using System.Drawing;
using System.Drawing.Imaging;
using System.Threading;

namespace OxyPlot2
{
    class Program
    {
        static Bitmap GetBitmap(System.Windows.Media.Imaging.BitmapSource source)
        {
            Bitmap bmp = new Bitmap(
              source.PixelWidth,
              source.PixelHeight,
              System.Drawing.Imaging.PixelFormat.Format32bppPArgb);
            BitmapData data = bmp.LockBits(
              new Rectangle(Point.Empty, bmp.Size),
              ImageLockMode.WriteOnly,
              PixelFormat.Format32bppPArgb);
            source.CopyPixels(
              System.Windows.Int32Rect.Empty,
              data.Scan0,
              data.Height * data.Stride,
              data.Stride);
            bmp.UnlockBits(data);
            return bmp;
        }

        [STAThread()]
        static void Main(string[] args)
        {
            var plotModel = new PlotModel { Title = "OxyPlot Demo" };

            plotModel.Axes.Add(new OxyPlot.Axes.LinearAxis { Position = AxisPosition.Bottom });
            plotModel.Axes.Add(new OxyPlot.Axes.LinearAxis { Position = AxisPosition.Left, Maximum = 10, Minimum = 0 });

            var series1 = new OxyPlot.Series.LineSeries
            {
                MarkerType = MarkerType.Circle,
                MarkerSize = 4,
                MarkerStroke = OxyColors.White
            };

            series1.Points.Add(new DataPoint(0.0, 6.0));
            series1.Points.Add(new DataPoint(1.4, 2.1));
            series1.Points.Add(new DataPoint(2.0, 4.2));
            series1.Points.Add(new DataPoint(3.3, 2.3));
            series1.Points.Add(new DataPoint(4.7, 7.4));
            series1.Points.Add(new DataPoint(6.0, 6.2));
            series1.Points.Add(new DataPoint(8.9, 8.9));

            plotModel.Series.Add(series1);
            using (MemoryStream stream = new MemoryStream())
            {
                stream.Position = 0;

                Thread t = new Thread(new ThreadStart(() =>
                {
                    System.Windows.Media.Imaging.BitmapSource bmp = PngExporter.ExportToBitmap(plotModel, 800, 600, OxyColors.White, 96);

                    GetBitmap(bmp).Save("test.bmp");
                }));

                t.SetApartmentState(ApartmentState.STA);

                t.Start();
                t.Join();
            }            
        }
    }
}