There is a famous Greek textual variant in 1 Timothy 3:16, where the “critical text,” SBL TH NA28 reads

 Ὃς ἐφανερώθη ἐν σαρκί  “Who/he was manifested in the flesh.” (relative pronoun)

and the “majority text,” TR RP reads

θεὸϛ ἐφανερώθη ἐν σαρκί  “God was manifested in the flesh.”

The difference originally in the most ancient manuscripts was much less clear in appearance.  Because there was a custom of contracting or abbreviating sacred names and concepts, by shortening them to fewer letters and putting a line over the whole group of letters, as can be seen in the correction in Codex Claromontanus (D- 06).  (These contractions were called “Nomina Sacra” or NS for short.)  Observe that the two-letter NS for God ΘΕΟC, which is just ΘC with a line over it, as seen in the correction of Codex Claromontanus, looks very similar to the relative pronoun OC in Codex Sinaiticus.  Note that Sinaiticus did not have an overline originally, and a late third hand made a correction toward the majority text.

What could have contributed to the problem is that scribes such as the one for Codex A used a caligraphy type pen tip, which was wide in a down stroke and very thin in a horizontal stroke. Thus the cross-bar in the capital letter Theta, Θ, could be very faint and therefore look like a capital Omicron, Ο.

Codex Claromontanus (D – 06) below:

Codex Sinaiticus (01) below:

Codex Alexandrinus (02) below: