。
Add or remove buttons from the OE toolbar.
Literal
OE [genitive-の] toolbar [to-に] buttons [object-を] add/remove-do.
A terse instructional caption. ＯＥ refers to Outlook Express (a dated email client). The sentence uses dictionary form する at the end — typical of instructional manuals and UI labels, where polite form would be unnecessarily wordy. 追加・削除する packages two operations into one verb phrase using the nakaguro separator. Full-width alphanumeric characters (ＯＥ) are characteristic of older Japanese computing text.