Updated October 17, 2026. Quick answer: Code O is the catch-all code in column (f) of Form 8949. The instructions use it for an adjustment that none of the other codes explains. You enter the code in column (f) and the dollar amount of the adjustment in column (g), and column (h) then picks up that adjustment.
Read box by box from the IRS form text: Form 1099-CAP boxes 1 to 5.
What code O means
The instructions list code O at the end of the column (f) table. The row begins with the situation: “You have an adjustment not explained earlier in this column”. The instruction beside it is short: “Enter the appropriate adjustment amount in column (g). See the instructions for column (g).” So the table row for code O does not name a particular kind of transaction. It is the code for an adjustment that the codes B, D, E, H, L, M, N, P, Q, R, S, T, W, X, Y and Z do not cover. The instructions also point to code O in two specific places: a Form 1099-B or Form 1099-DA with the Ordinary box checked for a taxable contingent payment debt instrument subject to the noncontingent bond method, and the first row used to report a QOF investment or inclusion event involving section 1231 gains.
Most rows never need it. The table opens by saying “For most transactions, you don’t need to complete columns (f) and (g) and can leave them blank.” And the last row of the table says that if “None of the other statements in this column apply because you have no adjustments”, the instruction is “Leave columns (f) and (g) blank.”
What to enter in column (g)
Column (g) is the dollar amount. The instructions say “Enter in this column any necessary adjustments to gain (or loss). Enter negative amounts in parentheses.” They also tie the two columns together: “Also, enter a code in column (f) to explain the adjustment.” The form itself prints the same reminder over the columns: “If you enter an amount in column (g), enter a code in column (f).”
An amount in column (g) with nothing in column (f) leaves the adjustment unexplained. With code O in column (f), the adjustment is labeled as one that the other codes do not describe.
Using O with other codes
A row can carry more than one code. The instructions say “If more than one code applies, enter all the codes that apply in alphabetical order (for example, “BOQ”). Don’t separate the codes by a space or comma.” So a row with code O and code W would show as OW, not O, W.
When two codes sit on one row, there is still only one number in column (g). The instructions say “If you entered more than one code in column (f) on the same row, enter the net adjustment in column (g).” Their example is “if one adjustment is $5,000 and another is ($1,000), enter $4,000”. Other codes with their own pages include Form 8949 code B for an incorrect basis, Form 8949 code M for several transactions on one row, and Form 8949 code T for a wrong gain or loss type.
Where it goes next
Column (h) is figured one row at a time. The instructions say “Figure gain (or loss) on each row. First, subtract the cost or other basis in column (e) from the proceeds (sales price) in column (d). Then take into account any adjustments in column (g).” The form’s own column heading says the same: “Subtract column (e) from column (d) and combine the result with column (g).”
The rows then roll up to Schedule D. The instructions say “Include on your Schedule D the totals from all your Parts I and II.” and “Complete Form 8949 before you complete lines 1b, 2, 3, 8b, 9, or 10 of Schedule D.” The overview is on Form 8949 instructions.
Edition and line numbers
This page reads the 2025 Form 8949 and its 2025 instructions the current files irs.gov served on 2026-10-17. They are the forms for the tax year 2025 return, filed in 2026. Check the tax year printed on the form you are filling in and use the column letters and codes on that year’s form, because a different year’s form can word its columns and codes differently.
Common mistakes
- Leaving the code off. The form says “If you enter an amount in column (g), enter a code in column (f).” An amount in column (g) needs a letter in column (f).
- Putting a space or comma between codes. The instructions say “Don’t separate the codes by a space or comma.” Two codes on one row run together, such as OW.
- Reaching for code O when another code fits. The row applies to “an adjustment not explained earlier in this column”, so check the other codes first.
Related: Form 8949 instructions, Schedule D, 1099-B box 1d proceeds and 1e cost basis, and Wash sale rule.
Sources
- Instructions for Form 8949 (2025): https://www.irs.gov/pub/irs-pdf/i8949.pdf
- Form 8949 (2025): https://www.irs.gov/pub/irs-pdf/f8949.pdf
Read from the IRS documents linked above on October 17, 2026. General information, not tax advice. Your own facts decide the outcome, and a preparer, the account provider or the IRS is the right place to confirm anything consequential.