Quoted from Oracle documentation 8.1.6

If Oracle does not find an equal or larger set of contiguous data blocks, it coalesces any free, adjacent data blocks in the corresponding tablespace to form larger sets of contiguous data blocks. (The SMON background process also periodically coalesces adjacent free space.) After coalescing a tablespace's data blocks, Oracle performs the searches described in 1a and 1b again.

So basically Oracle now coalesce the extents automatically