Both #""_8^16"O"# and #""_8^18"O"# are stable isotopes, and both were originally formed in stars.
Formation of #""_8^16"O"#
When a star runs out of hydrogen to fuse in its core, it begins to collapse until the central temperature rises to #10^8color(white)(l) "K"#.
At this temperature, the α particles can fuse rapidly, and #""_8^16"O"# is formed in a "triple alpha" process.
#""_2^4"He" + ""_2^4"He" → ""_4^8"Be"#
#""_4^8"Be" + ""_2^4"He" → ""_6^12"C"#
#""_6^12"C" + ""_2^4"He" → ""_8^16"O"#
Formation of #""_8^18"O"#
In contrast, #""_8^18"O"# is formed mostly in massive stars.
The processes involved are
#""_6^12"C" → ""_7^13"N" → ""_6^13"C" → ""_7^14"N" → ""_8^15"O" → ""_7^15"N"→ ""_8^16"O" → ""_9^17"F" → ""_8^17"O"→ ""_9^18"F" → ""_8^18"O"#
Thus, #""_8^18"O"# is much less abundant than #""_8^16"O"#.
