Select a vase that is short and chunky for an oval arrangement. Although you can certainly purchase oval vases expressly for creating oval arrangements, any short container will do. Serving bowls, decorative glassware or recycled containers work well as vases.
Soak florist foam in warm water until it is saturated. Cut to the dimensions of the vase and place the foam in the bottom of the vase. Florist foam creates a stationary base for the arrangement and makes it easier to display blooms as desired. Tape the floral foam in place with a section of floral tape. Make and "X" over the top of the foam and secure to the rim of the vase. The tape will not be visible in the final arrangement.
Select background foliage or spiked flower stems to create the height of the arrangement. Generally, two to four tall stems are sufficient. Anchor the stems into the foam so they create an interesting spray at the back of the arrangement. Ferns or other lacy foliage work well, as do delicate spikes of flowers. Create the overall shape of the oval with light-colored flowers or foliage.
Place the focal flowers in the midsection of the arrangement. Two or three roses or other medium to large flowers create concentrated color. Fill in around the center flowers with smaller blooms in similar color. Gradually fade to the palest blooms around the outer edges. Add leaves to balance the arrangement.
Display as a centerpiece or as an accent to any room. Keep the florist foam wet to prolong the life of your display.