Assuming you don't want it to be resizable, it works if you wrap it in a Border with a Height that is equal to the Width plus the height of the title section.  For example:
XAML:
<Window x:Class="MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
        xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
        xmlns:oxy="http://oxyplot.org/wpf"
        mc:Ignorable="d"
        Title="MainWindow" Height="350" Width="525">
    <Grid>
        <Border Width="200" Height="224">
            <oxy:PlotView x:Name="plot" Model="{Binding Path=PlotModel}"/>
        </Border>
    </Grid>
</Window>
Code-behind for the Model object:
using OxyPlot;
using OxyPlot.Series;
using System.Windows;
/// <summary>
/// Interaction logic for MainWindow.xaml
/// </summary>
public partial class MainWindow : Window
{
    public PlotModel Model
    {
        get; set;
    }
    public MainWindow()
    {
        DataContext = this;
        Model = new PlotModel
        {
            Title = "Test",
            TitlePadding = 0,
            TitleFontSize = 24
        };
        LineSeries line = new LineSeries();
        line.Points.Add(new DataPoint(0, 0));
        line.Points.Add(new DataPoint(1, 1));
        line.Points.Add(new DataPoint(2, 2));
        Model.Series.Add(line);
    }
}
And this is what it looks like:

If you want to do a resizable version, then use the containing window's SizeChanged event, and re-adjust the size of the Border container in that event handler.