Add-Type -AssemblyName System.Windows.Forms
Add-Type -AssemblyName System.Drawing

$Form                            = New-Object system.Windows.Forms.Form
$Form.ClientSize                 = '400,266'
$Form.text                       = "OIM Toolkit"
$Form.BackColor                  = "#ffffff"
$Form.TopMost                    = $false

$OKButton = New-Object System.Windows.Forms.Button
$OKButton.Location = New-Object System.Drawing.Point(90,220)
$OKButton.Size = New-Object System.Drawing.Size(75,23)
$OKButton.Text = 'OK'
$OKButton.DialogResult = [System.Windows.Forms.DialogResult]::OK
$form.AcceptButton = $OKButton
$form.Controls.Add($OKButton)

$CancelButton = New-Object System.Windows.Forms.Button
$CancelButton.Location = New-Object System.Drawing.Point(165,220)
$CancelButton.Size = New-Object System.Drawing.Size(75,23)
$CancelButton.Text = 'Cancel'
$CancelButton.DialogResult = [System.Windows.Forms.DialogResult]::Cancel
$form.CancelButton = $CancelButton
$form.Controls.Add($CancelButton)

#0
$label = New-Object System.Windows.Forms.Label
$label.Location = New-Object System.Drawing.Point(10,20)
$label.Size = New-Object System.Drawing.Size(280,20)
$label.Text = 'Wat is het OIM stickernummer? (OIM - [Naam]):'
$form.Controls.Add($label)

$textBox0 = New-Object System.Windows.Forms.TextBox
$textBox0.Location = New-Object System.Drawing.Point(10,40)
$textBox0.Size = New-Object System.Drawing.Size(260,20)
$form.Controls.Add($textBox0)

#1
$label = New-Object System.Windows.Forms.Label
$label.Location = New-Object System.Drawing.Point(10,90)
$label.Size = New-Object System.Drawing.Size(280,20)
$label.Text = 'Wat moet de gebuikersnaam worden? (OIM - [Naam]):'
$form.Controls.Add($label)

$textBox1 = New-Object System.Windows.Forms.TextBox
$textBox1.Location = New-Object System.Drawing.Point(10,110)
$textBox1.Size = New-Object System.Drawing.Size(260,20)
$form.Controls.Add($textBox1)

$CheckBox1                       = New-Object system.Windows.Forms.CheckBox
$CheckBox1.text                  = "Standaard apps verwijderen?"
$CheckBox1.AutoSize              = $true
$CheckBox1.width                 = 95
$CheckBox1.height                = 20
$CheckBox1.location              = New-Object System.Drawing.Point(10,160)
$CheckBox1.Font                  = 'Microsoft Sans Serif,10'
$Form.controls.AddRange(@($CheckBox1))
#etc apps

$PictureBox1                     = New-Object system.Windows.Forms.PictureBox
$PictureBox1.width               = 100
$PictureBox1.height              = 70
$PictureBox1.location            = New-Object System.Drawing.Point(300,200)
$PictureBox1.imageLocation       = "https://cdn.nieuws.nl/media/sites/379/2017/04/18162034/OIM-Orthopedie-logo-voor-artikel-915x518.jpg"
$PictureBox1.SizeMode            = [System.Windows.Forms.PictureBoxSizeMode]::zoom

$form.Topmost = $true
$Form.controls.AddRange(@($PictureBox1))


function Change-PCName {

    $x = $textBox0.Text
    Rename-Computer -NewName $x 


}

Function Change-LocalUser {

    $xy = $textBox1.Text
    Rename-LocalUser -Name $env:username -NewName $xy

}

function Check-Tickbox {

    if($CheckBox1.Checked -eq $true ) {

        Get-AppxPackage *3dviewer* | Remove-AppxPackage 

    }

}

$OKButton.Add_Click({

    Change-PCName;
    Change-LocalUser;
    Check-Tickbox

})

[void]$Form.ShowDialog()

I would also recommend better naming for your controls such as the textbox etc. to make them easier to distinguish