Need more information, such as the depth of the pool. Is the surface 18' by 5' rectangular?
18x24x4.5x7.48 US gallons
You need more information to answer the question. One is that there are no units associated with the measurements. The other is that for an oval pool, it is necessary to know which of the two measures are concerned with the oval and which is the depth.
Go to the web link below to get the right equasion
About 18,000 gallons.
No, you cannot convert a round pool into an oval pool. The measurements would be off and you would be unable to move the pieces of the round pool to form an oval.
15' x 24' oval pool with 54" wall has 8,823 gallons of water.
20,500 gallons
Assuming the pool is rectangluar (and not an oval or some other shape), then the volume is 2,875,392 cubic inches. There are 231 cubic inches in a gallon, so the pool holds 12,447.58 gallons of water.
a lot
http://www.havuz.org/pool-calculators.htm try this link
I am not familiar with oval pool deck plans, but would think that they are just as good as a round or rectangle deck plan. Deckplans.com offers a wide variety of deck plans and gives you an estimate of how much it will cost to build the deck.
Due to the differences in engineering and the parts involved, there really is no way to convert an oval above ground pool into a round pool.