In Jastrow, vol.1, pg 132, it gives several meanings for "אתא" depending on the context. They are: to join, to include, to arrive, and to occur to.
In Jastrow, vol. 2, pp. 1083-1084, it explains that the root of "על" is "עלל". Again, there are several meanings dependent upon the specific usage. They are: to bring about, to go about, to attend to, to be liable or likely to, to enter, to attend, and to search for.
In the context of your posuk, VaYeshev 30:16, the Hebrew is:
וַיָּבֹא יַעֲקֹב מִן-הַשָּׂדֶה, בָּעֶרֶב, וַתֵּצֵא לֵאָה לִקְרָאתוֹ וַתֹּאמֶר אֵלַי תָּבוֹא, כִּי שָׂכֹר שְׂכַרְתִּיךָ בְּדוּדָאֵי בְּנִי; וַיִּשְׁכַּב עִמָּהּ, בַּלַּיְלָה הוּא.
In the context of "על", Onkelos would be read, "And Yaacov 'entered in' (or 'came up', depending on the the elevation of the field as compared to the tents.) from the field, in the evening, and Leah went out to greet him and said, "he will 'look for' me (or he 'will attend to' me)...".
In the context of "אתא", Onkelos would read, "Yaacov 'came from' or 'arrived from' the field, in the evening, and Leah went out to greet him and said, "he will 'look for' me (or he 'will attend to' me)...".