The Join-OxfordComma command can provide you a comma separated list using the Oxford comma and either 'and' or 'or'.

Join-OxfordComma.ps1

function Join-OxfordComma {
    [CmdletBinding(DefaultParameterSetName='And')]
    [Alias('jox')]
    param(
        [Parameter(Mandatory,ValueFromPipeline)]
        [string[]]$JoinList,
        [Parameter(ParameterSetName='And')]
        [switch]$And,
        [Parameter(ParameterSetName='Or')]
        [switch]$Or
    )
    begin {
        $List = [System.Collections.Generic.List[string]]::new()
        $Type = if ($PSCmdlet.ParameterSetName -eq 'And') { 'and' } else { 'or' }
    }
    process {
        foreach ($Item in $JoinList) {
            $List.Add($Item)
        }
    }
    end {
        switch ($List.Count) {
            1 { $List }
            2 { '{0} {1} {2}' -f $List[0],$Type,$List[1]}
            default {
                '{0}, {1} {2}' -f ($List[0..($List.count-2)] -join ', '),$Type,$List[-1]
            }
        }
    }
}