The only free and comprehensive online etymological dictionary of the Spanish language
o (1) (Conjunction) "or"

10th cent. From Latin aut 'id.' From either *aut-i 'id.' in Proto-Italic. From Proto-Indo-European *h2eu̯ "away."

Romance: Western Vulgar Latin: Asturian o, Portuguese ou, Galician ou, Catalan o, French ou, Italian o ; Eastern Vulgar Latin: Romanian au

Italic: Oscan aut "or," Umbrian ute 'id.'

Indo-European: Celtic: Old Irish úa "away," Old Welsh o 'id.,' Middle Breton o 'id.;' Balto-Slavic: Old Church Slavonic u- "from," Russian u- 'id.,' Polish u- 'id.,' Slovene u- 'id.,' Old Prussian au- 'id.,' Lithuanian au- 'id.,' Latvian au- 'id.;' Hellenic: Ancient Greek αὖ (aû) "again;" Indo-Iranian: Sanskrit áva "off," Old Avestan auuā 'id.'
o- (1), op- Prefix "power to help"

Not productive in Spanish. From Latin ops 'id.' From Proto-Italic *opi- 'id.' From Proto-Indo-European *h3ep-i̯- "force." From *h3ep- "to work," "to force."

Indo-European: Hellenic: Ancient Greek ὄμπνη (ómpen) "corn;" Indo-Iranian: Sanskrit ápnas- "property," Young Avestan -haṇt- "rich;"
O (2) f. (Noun) "Virgin Mary;" any of the dates of the Advent Antiphons but especially "December 18th"

As the name for the Virgin Mary or the days of the O Antiphons in Catholicism, it comes from the frequent interjection O in the liturgical readings.

Responsible for the surname de la O.
o- (2), ob- Prefix "toward"

From Latin ob-, a grammaticalization of the preposition ob 'id.' From Proto-Italic *op 'id.' From Proto-Indo-European *h1opi̯ "at." Perhaps an ablaut of *h1epi̯ meant "on."

Italic: Oscan úp "at," Venetic op "from"

Indo-European: Balto-Slavic: Lithuanian api- Hellenic: Ancient Greek ἔπι (épi) "on," Mycenaean e-pi- 'id.;' Armenian: ew "also;" Indo-Iranian: Sanskrit ápi "by," Avestan aipi 'id.'

On the word *h1opi̯, Sihler (1995) writes, "Some see in these forms various cases of an otherwise lost root-noun meaning literally 'back.' The 'on' sense may have something to do with pack animals or riding."
objetivo m. (Adjective, Noun) "objective"

18th cent. An adjectival from objeto.
objeto m. (Noun) "object"

15th cent. From Latin obiectus 'id.' From ob- "toward" (see ob-) and iacere "to throw" (see echar).

Romance: Western Vulgar Latin: Portuguse objeto, French objet, Italian oggetto ; Eastern Vulgar Latin: Romanian obiect
obra f. (Noun) "work," "labor"

13th cent. From Latin opera "effort," from opus "work." From Proto-Italic *opes- 'id.' From Proto-Indo-European *h3ep-os 'id.'

Romance: Western Vulgar Latin: Portuguese obra, Galician obra, Catalan obra, French œuvre, Italian opera ; Eastern Vulgar Latin: Romanian operă ; Sardinian:òbera

Italic: Oscan úpsed "he worked," úpsannúm "working," Umbrian opset "worked," Vestinian ośens "he worked," Paelignian upsaseter (subjunctive) "he worked," South Picene opesaúom "to work"

Indo-European: Germanic Old Norse afl "power," Old Saxon ōƀian "to celebrate," Old High German uoben "to practice," Old English efnen "to perform;" Indo-Iranian: Sanskrit ápas- "work," Avestan huuāpah- "doing good work;" Anatolian: Hittite ḫāppar- "business"
obviamente (Adverb) "obviously"

From obvio and -mente, an adverb-forming suffix.
obvio (Adjective) "obvious"

From Latin obvius "on the way." From ob- "towards" (see ob-) and via "path" (see via).

Romance: Western Vulgar Latin: Asturian obviu, Portuguese óbvio, Galician obvio, Catalan obvi, French obvie, Italian ovvio
ocho (Cardinal Number) "eight"

13th cent. From Latin octo 'id.' From Proto-Italic *oktō 'id.' From Proto-Indo-European *h3ḱtéh3 'id.'

Romance: Western Vulgar Latin: Asturian ocho, Portuguese oito, Galician oito, Catalan vuit, French huit, Italian otto ; Eastern Vulgar Latin: Aromanian optu, Romanian opt ; Sardinian: oto

Indo-European: Celtic: Old Irish ocht, Old Welsh oith, Old Breton eith, Cornish eath; Germanic: Gothic ahtau, Old Norse átta, Old High German ahto, Old Saxon ahto, English eight; Balto-Slavic: Old Church Slavonic osmь, Russian vósem', Czech osm, Polish osiem, Slovene ǫ́səm, Lithuanian aštuonì, Latvian astuôņi; Albanian: tetë; Hellenic: Ancient Greek ὀκτώ (októ); Armenian: utʿ; Indo-Iranian: Sanskrit aṣṭā́, Young Avestan ašta; Tocharian: A okät, B okt