The original design of the 737 did not have ovoid inlets.
You can see regular circular nacelle inlets on the 737-100:
https://secure.boeingimages.com/archive/Boeing-737-100-2JRSX...
and on the 737-200 here:
https://secure.boeingimages.com/Assets/V2/jrZYYwCXohU1rDmk4e...
The ovoid inlet design was introduced with the 737-300, when the engine was changed to the CFM56.