I would plant a hedge around it.

You can paint it dark green while you are waiting for the hedge to grow. It seems to me ( if not anybody else) that things painted dark green appear to be less prominent.

Put up trellis work around it and grow a variety of climbing/rambling shrubs.

If you put Hide oil tank into the white search box top right you will see links to how other members have coped with the problem over the years, hope it helps

I agree with Loosestrife2, mine is a large metal tank and I grow Jasminum nudiflorum, Lonicera periclymenum and a variegated ivy up the trellis. I regret the ivy, though, it's too vigorous.

All of the above are good suggestions but remember to leave good access for the delivery man with the oil hose!